My riding club does 2/3 each year and depending on where you live, I know Stockland Lovell, near Bridgwater and Pontispool, near Taunton both hold regular camps
Funny as I was wondering the same thing today myself and would love to do one.
The only one I know of is Chislehurst and District RC one held at Chelsfield Equestrian Centre near Orpington. Haven't checked it out for years but generally was quite novicey, have a go type thing.
West Sussex riding club are running 2 this year - one in May, the second in August.
Its a great club - lots of friendly people and some very knowledgable people to train us. I'll be there too