I visited RAF Cosford last year and found them very helpful and friendly. Our club has the crown indemnity and we were only charged around £10 because we were on a "navigational exercise" (we navigated to find Cosford at least!). It was then a short walk to the RAF Museum which was well worth a nosey around.
Our club uses RAF Henlow as its preferred place for training - great friendly little airfield with some good deals, but make sure you get good PPR before flying in unless your club has an arrangement.
On the other hand, we wanted to visit Middle Wallop to go to the Museum of Army Flying and they were not half as accomodating. The landing fee was in the region of £30 and they wouldn't budge on it. They wanted insurance faxing before hand and all sorts of other things, so we gave them a wide berth!
Plan is to try Yeovilton next....!