here in London a mid-level engineer, which I assume is the bottom half of senior software engineers, are typically paid £50-£60k/year in startups, which usually have poor pension schemes. £50k is about $60k and you already start paying for 40% tax for the top £10k of your salary.
You can get more if you go to big companies but then so can you in the US.
If you plan to run a start-up, and you can have remote first organisation (physical co-location is not required by say client attendance or manufacturing process) then do so!
And if you do stay in London, compete with banks for best talent with your wallets, not ping-pong tables.
I don't know the details of the costs of living and all the expenses. Maybe SV is better but I just want to say that it is not twice as better, contrary to what some comments would have you think.
The cost of living in SF is 1.4 times higher but the wage is 1.75 times higher. Note this number is assuming you are renting, and not considering electronics which is a lot cheaper in the US.
The house price in London is still more expensive, and if you are a senior developer in your 30s that's probably more relevant than renting price.
Factor those in, as a software developer, in SV you are easily 30-50% better off than London, the best paid city in Europe. The pay gap between Europe and US do exist, Silicon Valley is the technology centre of the world, Europe isn't.