I seriously doubt it. Is London even more expensive than Mountain View? They could also move to a different european city, wages are low everywhere in Europe. Ironically, the largest engineering site of Google in Europe is Zurich, where employees are as expensive as in Silicon Valley.

Google has about 40000 engineers. If they paid $70k less per year per engineer (median total compensation for a Google engineer is probably around $200k), that's a saving of about $3 bilion a year.

I cannot understand why so few US companies open engineering sites in Europe, and when they do, they are much smaller than in the US. It's really easy to get good engineers in Europe.