Well, it's complicated
London occupied a pretty defined central area until quite recent times (c1800). For example Kensington Palace was a Royal 'out-of-town' summer residence, and the adjacent Kensington Gardens, now half of what is commonly referred to as Hyde Park, was where the king and his entourage used to go and hunt deer! A further example is Portobello Road, named after the Battle of Portobello, and now famous for it's street market, which used to be a simple farm-track connecting Notting Hill Gate (the 'Gate' was derived because it was the site of a toll-gate on the road from the west into London*). That track led from the Gate up to a farm about two miles north called 'Noten Barnes' [i.e. 'barns']. Located on a hilltop, the name for the area evolved over time into Notting Hill.
As London grew in wealth, so did the likes of population density. Houses were heated via coal fuelled fireplaces. Usually at least one in every room. Houses were big, and rooms were small enough to facilitate a simple fireplace heating them, so a middle class house might have say 8 - 12 fireplaces, and grander place might well have 20+. This of course was well before the advent of smokeless fuel, and so London fast became something of a victim of it's own success. The air was very polluted, and in winter it was commonly shrouded in a 'pea-soup' fog, or more literally smog. This is where the term (later borrowed by a historic US rain-wear company) 'London Fog' came from. Maybe you've seen the sort of conditions portrayed in Jack the Ripper (etc) movies, where you'd be out on the street and not be able to see your own hand extended before you.
So what happened was there was some kind of societal movement from the 1830s. Some of the larger family estates (Grosvenor, Cadogon, Portman, deWaldon etc), begun buying whole fields, whole farms, out in what is now 'Zone 2'. There they built magnificent squares and white-painted pillar-fronted terraces of grand houses. These were pitched as being healthy areas, more suitable for the rapidly growing middle classes, with garden squares and communal gardens, where families could enjoy space and fresh air, away from the dirt and overcrowding of the centre of town. And the people flocked to them, and hence rose neighbourhoods like Knightsbridge (and yes, that was named after a bridge across a tributary of the Thames used at least in part by knights heading to Hampton Court Palace to the west!), Marylebone, Kensington, Notting Hill, and so on.
But World War One changed everything. In 1913 you might be a well-to-do middle class family occupying a house like one of these - Stanley Gardens, W11.
... with parents, perhaps 6 children, and 3-5 staff. Maybe 5-6,000 square feet each, and backing onto small private gardens, that in turn back onto enviable communal garden squares. The domestic staff were in the front of the line that got drafted to the front-line, and a huge proportion never returned. A lot of young men, sons of the household who otherwise might have expected to inherit the home were enlisted as officers. Ditto as to their fate.
The result post-WW1 was the surviving remains of families occupying huge houses, which were under-staffed. Suddenly areas such as these started to become somewhat less attractive as a reduced family with less or no staff, and less income, were simply not capable of running them. WW2 was the follow-up blow that changed everything. A great deal of family wealth was destroyed, as were those young enough to earn it. Longer established 'suburbs' such as Kensington and Knighbridge were less impacted, as by then they were probably home to long-established inherited wealth. More recent aspirational suburbs such as Notting Hill were hammered hard and by the late 1950s they were run-down ghettos where no polite person would dare to venture.
The UK begun a policy of mass-immigration of labour from the colonies. Many people from the Sub-Continent emigrated to the northern mill towns of Bradford, Coventry, Birmingham, and so on. West Indians who were coaxed with jobs as train drivers, postmen, bus drivers and so on seemed to congregate in west London, especially centred on Notting Hill. This saw the rise of rogue landlords like the notoriously violent Rachman http://en.wikipedia.org/wiki/Peter_Rachman
" onclick="window.open(this.href);return false; It's after him that the British terms rachmanism, and 'a rachman landlord' are coined.
He bought up those huge houses, and housed maybe 20-30 immigrants in each, in slum conditions. A lot of the rights and protections you see today in modern tenancy law, tenancy agreements (even here in SG) likely derive from this one monster of a man.
Places like Notting Hill became essentially no-go zones. No sane person would venture north of Notting Hill Gate, even into the late 60s. Gradually, almost street by street, that boundary was progressively pushed further northwards. New housing laws and protections finished off the likes of Rachman. Aspirational but poor workers saw these magnificent streets of houses, and so there came a market for splitting up the huge old houses into flats that people could afford. The business opportunity was so compelling that hordes of building contractors were brought in from all corners to carry out such conversions. Many of them were from Ireland and were semi-skilled. There were almost no building regulations/codes so invariably the 'conversion' into flats was done as cheaply and hence shoddily as might be expected when a quick profit was the only goal. This is where the expression 'Gerry-built' derives from, meaning done badly/done cheaply. Gerry being a euphemism for your then average working-class Irishman.
By the time I moved there, the Front-line had pushed northwards to Westbourne Grove. My first home, as the owner, was the ground floor of a previously grand house located about 400M to the north. Even 25 years ago you had to keep your wits about you going home from work up those final few streets. Maybe akin to heading north above 96th (IIRC?) or 110th streets in Manahttan. The buildings were glorious from the street, but the internal 'build-outs' were usually of terrible quality. At the place a few streets away that is now 'my home' I had to remedy that by taking out all the internal walls and ceilings and pretty much starting again from scratch, in strict compliance with today's onerous and expensive Building Regulations.
So that's a double-espresso fuelled potted history of aspiration, deprivation, Gerry-built opportunity grabbing > final progressive gentrification and a phoenix-like rise from the ashes.
I'm sure there are parallel histories in other cities. I had the formative experiences of staying in the rather notorious 'Rex and Stiffles' hotel in Bombay in 1983, and a few years later the perhaps even more notorious Chungking Mansions (ha, what an ironic name!) in Hong Kong...
Notting Hill has now changed so much (I described this previously). The buildings all look pretty much the same from the street but the people who live there seem to have changed beyond recognition. The remaining West Indians who arrived in the 60s together with their [Notting Hill] Carnival must feel the same when they choose to take the money and move on.
* I love these old names hehe... Shepherd's Bush centred on Shepherd's Bush Common, was where farmers from the 1600s onwards would drove their flocks in from sheep-country in the west. That was their last RnR/overnight location before they'd continue onto Smithfield (meat) Market in the City.