Those companies already exist in London and they give a haircut of 30% over US salary practically as a matter of policy. Why would they pay more than they need to in the abysmal UK market?
I know someone who is moving from NYC to the UK to run an engineering team at a FANG company. He'll want to hire the best engineers and be willing to pay for them. And I know he wouldn't have taken that job if it didn't come with a big pay increase.