^I think a bit less London centric but you've got to consider that London will always carry a certain appeal, it's got status, top jobs, attracts a lot of foreigners, massive leisure scene etc.
Flats are attractive because of the cost / proliferation in certain areas, most buyers over a certain age probably want a house already, unless they have some fancy apartment building with onsite facilities like concierge, gym etc. That said, maybe you'll have an army of people who say actually I no longer want a 1-2 bed flat in the centre I'll instead look for a house out in the sticks for the same money, because I'm not going to need to travel that much.
Not sure about gardens, generally people either make the garden a priority or they don't, regardless of where they work.
The family areas, basically what I'm saying is perhaps the 'commuter belt' expands. Traditionally you have the people in Surrey, Hertfordshire, Sussex etc commuting in to London, these are people who want bigger houses with gardens in nicer areas, but they are historically a bit constrained by needing to get into the capital. So they can't venture too far away and as house prices in Greater London have soared over the past 20 years or so more and more people have been pushed into those areas making them quite expensive in themselves. As I mentioned earlier I spent a long time looking online at houses in various areas (thousands of houses) but even the areas people talk about as areas 'Londoners move out to' can be prohibitively expensive even on a City wage if you don't have the huge equity to cash in from a London property. But now people may take the view that well, I no longer need to be within an hour's train ride to London. I'll go somewhere two hours, three hours away where previously there were very few commuters driving up prices. I currently live beyond the boundary of what many consider acceptable as a commute (4.5hrs round trip) but maybe these sort of places will hold more appeal in future.
Will be interesting to see how rail passenger numbers are affected in the medium-long term. Makes you wonder about HS2, doesn't it?