London is notorious for low SWEng salaries; you might not have any other choice than to contract somebody in Ukraine who didn't escape somewhere else if your own employees get higher paid remote gigs in the US...
I'm from Liverpool originally (live abroad now) and I have friends in Manchester earning £50-60k... in the North of England. That's a big wage in that area.
The stat you link appears to be for "IT" jobs across all of the UK. Restricting the search to "Software Engineer" and "London"[0] bumps the number up quite a bit to £72,500 as of when I checked.
Yeah, even both Google and FB pay engineers pretty terribly in London. I was once asked to move there, and I refused unless they accounted for cost of living changes. They didn't, so I stayed working at my original office.