Not sure about Google but other large/well-known tech firms seem to pay their UK engineers on par with their US ones based on speaking to my UK friends who work for SF-based tech companies.

I hear contracting for day rates in London is the route to ££ in the tech field over there (or working for a US-based company), but the salaries/equity on offer in the US even for non-rockstar positions dwarf what UK employers are able or willing to provide.

The UK massively undervalues and under-compensates its technical talent, it's a big part of why I left the country.