Welcome to the site.
I've spent 7 of the last ten years in Wiltshire and 'fell' type races are few and far between. Most of the ones I know are in Jan, Feb and Mar and tend to be called multi terrain or trail races. (Fjell being the old Norse for mountain, and a bit foreign to the Saxons down south)
This is a good one in Apr
http://www.chepstowharriers.org.uk/offa.htm
I've heard there's some good racing in Shropshire and the Malverns.
Google the running clubs like Cheltenham, Axe Valley runners and see what's on offer.