I have been a New-Yorker since 1969, but my first visit to London was in 1956 when I was a child, and here are some abiding memories of that wonderful city, and later opinions based on observations over many years of revisiting:
(1) The war damage -- there were still large areas even in central London, especially around Blackfriars where lots of the big and most tastelessly egregious office buildings are now, spoiling the view of St Paul's from the Thames, that were ruins and were being slept in by derelicts. What they (the Redevelopment Commission or whoever) did in the northern Cripplegate area at London Wall to restore it was horrendous, although lots of medieval and Roman stuff got unearthed. It has been improved since the London Museum moved there and the Barbican development was built in the 1970's (many people hate the latter, but I rather admire it, especially where it has been put over a series of moats and ponds, with the recovered Roman bits displayed). The problem is, and blame the politicians (esp. Labour Party), for putting resources into the Festival of Britain and things like that rather than into rebuilding infrastructure and reconstructing lower-class family housing instead of opting for the later mega-flat slums of the late 1950s and 1960s.
(2) The pea-soup fogs (London Particular) and the acrid smell of coal smoke (which you also got when you opened a window on a train, since they were still using steam engines then), from all the fireplaces that were soon to be banned in the city, hence only 'clean' fogs nowadays. Car exhaust smells far more offensive than coal smoke, although nobody has tried to link that to lung disease with as much fervor as the anti-tobacco people in our times.*
* A few years ago I was in St. Louis, Missouri, on business, and on free time went to visit the old train station, now a yuppie mall; it does, however, have a train museum. The second I walked into the place I smelled that train-engine coal smoke and had an olfactory orgasm -- humans are no way as sensitive as dogs to smells, but it is still a basic part of our brains that is often ignored. It aroused a slew of memories instantly, connecting through the medula or whatever, just as my first girlfriend's perfume would if I whiffed it now, even though I don't know what hers was by name. The smell of tobacco affects me the same way, because my grandfather, who reeked of it, was my most-loved person in childhood -- that's one reason that I've always failed in giving-up-smoking attempts. Most people say it stinks; it is incense to me.
(3) That horrible dirty-yellow sodium-lamp street-lighting that turned the whole sky orange but did little to light up the streets. Also, the road system out of London was really terrible -- a Sunday expedition on the A40 out Windsor-way, before England had any four-lane highways, was horrendous when it came to the evening rush back to London. There weren't that many families who had cars then, too, though lots of motorcycles with sidecars (under the Socialist government, your road/car license tax was based on the number of wheels you had! and three was considerably cheaper than four, hence also the popularity of 3-wheeled cars, which I never saw in any other country). Still, there were traffic jams. And lots of people just pulled off the road and brewed tea on flimsy tables with sterno-fired kettles while watching the traffic creep by.
(4) Anachronisms such as rag-and-bone collectors on horse carts, street criers ("cockles and mussels alive-alive-oh" if you know what I mean), onion sellers from Brittany on bicycles who provided huge strings of delicious bundled-up onions that would supply a household for three months (Mum would hang them up on a hook in the kitchen), cheap but really good antique (for me) stuff like war medals and bayonets -- German Iron Crosses were especially plentiful, being war loot -- possibly lots of it 'fenced', in Portobello Road and other places that are now tourist traps -- could go on and on with this, but it is all GONE now. (Portabello Road is not much changed, but the quality of the goods is considerably lower than it was.)
(5) No fast food. In fact no restaurants to speak of outside of London, except for the more famous inns and hotels. On the other hand, lots of tea shops and fish-and-chips places. Forget trying to get anything at all on a Sunday. The Wimpey (from Popeye the Sailor) hamburger chain was the first in Britain in the class of the original US 15-cent Macdonalds and White Castles (class in the meaning generic rather than qualitative -- they were all comparatively at the true junk food level and deliciously indigestible).
(6) PLUS SIDE:
(a) Public transport. Before Beeching dismantled the railway system except for the main intercity lines, one could go anywhere by train, and trains usually ran on time and didn't break down. Also the local public 'Green Line' buses (they didn't really have intercity bus lines of the Greyhound sort then) ran frequently and had that nice double-decker format so you could see what was going on en-route from the top level over the hedgerows.
(b) Prices. Everything except for electronics (TV sets, phones, radios, and collaterally, cars, dishwashers, refrigerators and things like that) was very cheap. You could have a very good dinner of 'home'-cooked roast beef with soup, veg, dessert, cheese, and so on for the equivalent, even considering inflation, of two Big Macs. A pint of beer was only sixpence (so I've been told, since I didn't start drinking until I was 14); a pack of cigarettes for a florin (ditto).
(c) Pristine countryside. While the Victorians had polluted certain areas with railways and their adjuncts, depressing acres and acres of grim row housing, and of course the horrendous factory areas to the north of London, there were no intrusive power-lines running all over the country, no cell-phone antennas broadcasting their microwave emissions all over everything in the neighborhood, no huge pine-forest plantations, grown for the sake of the paper and furniture/housing industries, covering the hillsides, no destruction of the hedgerows and small-field farming layouts in the interest of huge agribusinesses who need large flat areas like a Kansas cornfield to be profitable (and to hell with levelling old neolithic burial mounds and things like that in the process).
(d) Shillings and Pence. Minor, but telling point, is that the pounds/shilling/pence system was still in effect. Hard to do math in, especially percentages, but since decimilization how much English literature will now become partly denuded because nobody will know what a tanner is, a bob, or even tuppence? But with a half-crown, now just 12 1/2 p (a quarter to US audiences), you could hire somebody to take a message all the way from one side of London to the other then, without a tip! In 1956 there were still farthings in circulation, soon to be abolished as valid coinage -- a farthing, when there were 240 pennies in a pound, was worth a quarter of that, i.e., 1/960th of a pound (like the 'mill' still used in the US when charging for gasoline, that point-9 thing on the per-gallon price that makes you think it's cheaper). And the pound was worth about $5 US then; yet all you could buy for one farthing was a gumdrop; they were tiny bronzish coins with a grouse or some sort of game bird on the tails side. (Not that I ever knew what a sovereign, sesterce, sou or shekel was, and that didn't really spoil anything except for just having to guess at what the value was -- so maybe this doesn't matter -- we have the expression "not worth a sou" but that could have been a year's wages for a French peasant in 1200. Same thing will happen to inches and pounds in another generation or so.)
Modern buildings. As for the post-war architecture, I can only agree with Prince Charles that it is absolutely awful and should be torn down. However, on this point, much worse was done a lot later, and not to replace war damage, but out of insensitivity and economic greed, such as those horrible multistory car parks and red-brick shopping precincts in the middle of historic cities where perfectly good buildings were torn down for convenience; more of Historic Britain has been destroyed in this way than was ever done by the Germans. The worst thing about the post-war rebuilding was the total neglect of the existing infrastructure such as street layouts, and the lack of general planning schemes based on local demographics, i.e., neighborhoods.
Bus Numbers 9, 13, 74 -- are these magic numbers? Everything in my life back then depended in some way on those bus lines (no longer pristine with London Transport since they broke up the continuity of those lines and replaced some sections with single-deckers run by the driver with no conductor, no jumping on or off of an open platform at the back, and now even make you transfer in places rather than do the whole route -- then you were allowed to smoke in the top section, gasp gasp, but Jeeze at least give us unregenerate smokers SOME place to do it). Well, #9 went from Mortlake in the SW all the way through to St. Paul's cathedral and beyond, through Piccadilly and all that, by far the best bus route in London back from where I lived in Barnes then until the near collapse of the great Hammersmith Bridge cut out the weight of double-deckers there and forced an intermediate bus where the little old ladies hold up the thing interminably counting out their change to the driver before he can move on. #13 was the one that ran from Victoria Station to almost anywhere you wanted to go north, to the BM or Zoo. And #74 went from Knightsbride, via Primrose Hill, to Camden Town (my way of going to school in those days). Took about half an hour back then, from Harrod's to Regent's Park but I'd betcha it would be more like an hour and a half now considering how traffic-clogged the northern parts of London have become.
Back to mysteries: One of the appeals of the Golden Age of British mysteries, especially when it comes to London, is to perceive that city in a different era, just as one appreciates Van Dine or Ellery Queen when it involves 1930's New York City. Of course, one can argue that Edwardian London (actually George-the-Fifthian, but that doesn't scan properly) and the New York of Jimmy Walker were also totally transformed from prior ages such as the early 19th Century, the pre-railway days of Pickwick and Poe -- no doubt, older people of their time complained of that just as much. The mysteries written in the post-War years, 1945-1955, have a certain appeal that cannot be matched now, although the Golden Age of Detection was over. This period saw the dawn of James Bond and the twilight of Christie and Carr. Oddly enough, Fleming retained the optimism of the pre-War years, kept up the Richard Hannay tradition (this was before Le Carre revived the noir stuff of the early Ambler sort), while Christie and Carr expressed mortification and depression. Agatha waxed nostalgic, and John went haywire, writing some of his most bitter books when not delving into the past.