I've never actually been but have phoned up several to find out and it was never simple. The last one was Cosford, about 6m ago, and they wanted 24HR PPR by fax, with copies of insurance documents. I could not be bothered and arranged to meet the other party near the much more pleasant Welshpool instead.
I don't know why they don't just open up, and make a useful £10-£20, not to mention fuel sales, each time somebody lands there. There seems to be zero activity on most RAF fields that I have flown near.