You have to compare apple to apple - eg Google/FB salary in Bay Area vs Google/FB salary in London. Skills of average engineer in Bay Area (due to magnet status of the area attracting best and brightest) is significantly above skills of average engineer in pretty much any other location and comp reflects it.

For Google/FB difference in comp by location is there but not as dramatic as difference in average salary for all engineers per location may suggest.

To enjoy social life in Paris, Rome, Berlin or Hamburg you need to speak local language (otherwise you just meet the same expats) which is not problem in London as everyone speaks English.