I think that's an interesting thought in general, but what used to be true still is today. Europe in general, and the UK in particular, has significantly weaker free speech guarantees than the US does. This manifests in quite a few ways, from "hate speech" to "right to be forgotten" to "superinjunctions" to prohibitions against publishing the name of certain people accused of criminal acts.
Moving to Europe would oblige the WMF to follow these restrictions. How would that be avoided? If anything, Canada might be a better option in that regard, but still far from perfect.
Also, I'd dispute that London is inherently more creative and vibrant than SV. Some of the most brilliant and creative people in the world live and work in Silicon Valley.
Todd