The most common spots for EU datacenters are Frankfurt (Germany) and Amsterdam (Netherlands). London is somewhat behind these two and used mainly when you also want a decent US connection while sacrificing a bit of Eastern Europe connection.
So I really hope it's one of the first two.
1) They already have an extremely good infrastructure.
2) They have a TON of trained people living around them for if SE needs more server techs.
3) They have the best ping for overall Europe (including Eastern Europe and Scandinavia)
