Median pay for a dev in London is about £60-65k(a out $87,000). This isn't that high given the cost of living in the city, with your average shared flat with a few roommates going at about £1k a month nowadays. Add in that you'll be hitting the marginal tax rate of 40% plus 9% student loan tax and about 8% national insurance (ok income under 4k a month) and you don't end up with that much to live on.

The average university grad would be better off in law/finance/medicine by income in London. This isn't to stay the top software Devs don't get paid a lot, but it's a minority compared to the legions of high paid people in finance in London and the surrounding industries.