Not London....who hasn't been to London???!!!! There is nothing particularly novel about going to London.
I still vote Sheffield. Its really the natural choice, being the home of the PGF. What good is a meet up without the forum administrators, without whom none of this would be possible??! Besides that, I think its worth the excuse to get out to a different part of the country and see another town. Yorkshire is a particularly beautiful part of England, and altogether different in feel and character than London. And it has better pubs, IMO, better pub food, friendlier pub inhabitants, better football teams, and a MUCH more pleasant regional accent than the regional accent of the Home Counties. Yorkshire is the absolute gem of England.
This might be the only reason some of us would have to get to Sheffield, so why squander it? Broaden your horizons! Spread your wings! Don't be London-ist!