My first tech job in London was at a medium-sized (~300 employees) tech company HQ'd in Seattle, with London being 2nd largest. Not long after I joined, one of the team leads left for a largish fintech, and slowly brought the rest of his team with him over the next couple of years. Despite this constant brain-drain, management refused to address the salary issues.

After a couple of years, I left to go there too, with a ~60% raise(!). I learnt from my manager, who left a month after me to the same fintech, that HR only benchmarked against "comparable companies in the same area", meaning west London suburbs, and ignored the pull of Central London finance, despite literally dozens of people leaving to go there. You gotta be realistic against who you're competing against, and thanks to how mobile tech professionals are, European companies are competing against not just their area or city, but often against Silicon Valley.