Technically, Mozilla Denmark is the highest level subsididary in Europe, so all offices in Europe are offices of that Mozilla Denmark (it says it on the Mozilla Paris office, it's pretty funny) - but it's much more metaphorical than official. It's a matter of where will European Mozillians go to meet up, work together, etc. I don't think London has that appeal; most non-UK Europeans don't consider London to really be Europe - hell, Londoners don't consider themselves to be european.