How much you pay in London really depends on how/where you want to live. It's far cheaper if you're happy to spend an hour or more commuting each way.

I need to earn £50k to cover just childcare costs and the mortgage (looking forward to both of those going down soon). I could sell up and buy a similar property 5 miles away and those costs would both halve. I could sell up and buy a similar property 10 miles away and have no mortgage at all. Neither of those would make us happier as a family (no matter how much I'd love to have no mortgage); we want to live where we're living now, so I just get on with it.