I'd vote for London, but that's just because I'm lazy and selfish.
Perhaps it would be easier to see how many people interested in coming to this event live closest to which proposed location, and take it from there. It might also be possible for some members in the location we decide on to take on some people to stay with them? - the rest of us could camp out somewhere nice... If we're taking our Amigas (computers) a secure location to keep the kit would be of essence