World Pea Shooting Championship
Witcham, near Ely, Cambridgeshire
Next being held on 13 July 2013.
Quite a few places hold a Scarecrow Festival, including Langwathby (Cumbria), Harpole (Northants) and Hayling Island (near Portsmouth).
National Giant Vegetable Championships which seem to always be a part of the Royal Bath & West Show at the end of August.
Cow Dung Festival, County Mayo, Ireland
But then there's a Cow Dung Festival in Switzerland as well!
British Beard and Moustache Championships
Held in Brighton in September 2012.
There are some brilliant pictures here. The 2013 World Championships will be held in Germany.
World Snail Racing Championships
Congham, near King's Lynn, Norfolk; August 2012
World Stone Skimming Championships
Easdale Island, near Oban, Argyll
Next on 29 September 2013.
World Custard Pie Championships
Marden, Kent; September 2012
** I was going to say England, but then realised there are Irish and Scottish entries in the list.