I always recommend Dragonmeet, but that might be a little far away.
Games Expo in Birmingham is really good, although not a purely RPG con. It is really well organised though and lasts a couple of days.
I've just come back from Indiecon which was a lot of fun but right down the south. Mind you, may be worth the trip if Indie type stuff is more your thing, which I'd expect is oftne the case for a lot of us on this board.
There is a con in Edinburgh (Continum I think, Con-something anyway). I've not been but it seemed well supported and solidly organised.
However, I'm told that Gaelcon or indeed any Irish games convention are the best of all.
The 'UK Roleplayers' forum has a good listing of UK coventions which might help.
It will give me a good idea as to wether i actually like conventions without having to spend a butt load of money traveling millions of miles to christchurch etc