If it's in Europe, it should be in London.
Can you please explain why?
I am french, I'm the #3 most prolific writer on this forum and as such I am offended by this proposition.
Sorry, didn't mean to offend anybody. London is convenient for conventions because it's the most densely populated area in Europe (so there are usually lots of potential attendees close by), it has direct connections from/to every European airport and it's been my experience with past conventions that you get the best attendance there because of those factors.
Note that I'm in Switzerland, so London would be further for me than it is for you, but it's still pretty convenient to shuttle out of Zurich Kloten.
But that's just my experience, if it's in Europe and it has an interesting enough roster of speakers to warrant traveling, I'll attend wherever it is. Ultimately it's going to depend on where we can find somebody who has the actual time, money and energy to organize a convention.