Any of the Lakeland Trail events - just google Lakeland Trails. Very popular, well organised, slick events.
Coniston 14 road race - April (I think). One of my favourites.
Check out the LDWA challenge events on their website - Grasmere in April, Borrowdale in June or the brilliant 4 passes challenge in October (not that I'm biased!)
Langdale marathon/half marathon