Number one factor in London for cost is housing. Frankly take this out, and you could live in London pretty much as cheaply as anywhere else in the UK. To take two ridiculous extremes, you could look at Harrods and say this is London prices - or you could go to Tesco or even the poundshop and see something more realistic.

Plenty of people in London with a lot of money, and there will be shops catering for this, but of course there are plenty of normal people with more modest wages, and there are normal supermarkets, markets etc to cater to this end.

Housing though is ridiculous. Compare to my town, the "best" area of town, with sweeping parkland, private prep schools a grammar school, trees galore.......you get the picture. You could buy a new 4 bed detached home here, in middle class seclusion on it's private culdesac for a bit over £200,000.
Way over my price range, but the nearest equivalent quality of area in London, I would say is St Johns Wood or Wimbledon Village. I don't need to compare prices there to make a point.

Oh and most areas in this town, I would say I would be pretty safe walking on my own after dark, even on the estates. Centre of London isn't as unsafe as people sometimes think, but all the same..........and the suburbs of London, you'd have to be crazy to do that in some areas.