We're members of the Caravan and Motorhome Club - we re-joined after a gap of some 20 years. Their big sites are usually very well maintained and the certificated locations (5 vans only) vary, but are often in nice rural spots. We're very happy with it and have stayed in some lovely places. The information in the handbook and online is very helpful in choosing suitable sites.
The Camping and Caravanning Club has similar facilities (DH's sister and husband are members of both clubs) but you're more likely to encounter tents, I think.
There are also places and organisations that allow overnight stops rather than long stays - useful to stop off if travelling a long distance - for example, BritStops.
Hope this helps a bit. Enjoy your motorhome!
