i know it's been a while since you posted this but i thought it might be a good idea to post what i know since the conventions tend to be yearly.
In England there are at least 4 to 5 conventions that are worth going too for anime, there are a few others but they usually get boring quite fast.
1) Amecon
2) Kitacon
3) Ayacon (will be hosting their first cosplay ball)
4) HyperJapan (anything Japanese can usually be found here.)
I remember there being 1 more but i really can't remember the name...