It really depends on your definition of crap. London dev salaries range from £24k for absolute juniors to £120k+ for the top 2%. Contractors fare substantially better. In my recruiter days I placed a Python developer who had experience with a particular trading system on a contract with a large trading firm. His contract was £1,100 per day for 6 months.
Keep in mind the other notable differences between here and the US such as free health care, a larger number of paid holidays etc.
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.
I disagree. We have a handful of senior devs being paid in excess of £70k. There are plenty of non-finance companies paying similar salaries. £80k-£90k is relatively common for a senior developer in London.