I’ve also lived in the UK and US, though did not work for a FAANG in the UK.

By the exchange rate right now, 50k GBP is around 65k USD. If you want to live within (say) an hour reach of an office, you’ll have a much easier time doing that in the London metro area with 50k than you will in the Bay Area with 130k. This is before you even consider taxes and healthcare. High quality groceries also substantially cheaper in the UK. A new wildcard is the price of energy, which looks to have gone up considerably in the UK since I left, though.

Neither would be a comfortable position, and you wouldn’t be in danger of being able to buy a house absent other income. All in all though, if they were my fixed options, I’d take the money in London.