Yorkshire Riding Centre do them and are pretty good from what I've heard. Also further South some of the pony clubs do 'residentials' for adults too and from what I remember you dont have to be a member (was invited on one and am not member)
Stour Valley Riding Club do them, we had two last year for all abilities, we had a great time. Lots of wine in the evenings too, works out about £155 for the camp which is three days training then mini comp on the fourth day and that includes staying in the house and food.
You do have to become a member though.
I'm a member of South Oxfordshire and Naphill Riding Clubs,
SORC offer a 3 day camp at Milton Keynes Eventing Centre in July.
Naphill offer a 2 day camp at MKEC in May and also a 5 day camp at Moreton Morrell in Warwickshire in July.