I'd favour a fixed fee of ~£30 (inc. handling and parking for a day or two) for a diversion to any regional airport. It's not enough to influence a safety decision, but some recompense for the airport operator.
A pan shouldn't be necessary. The ATC officer should decide to accept a diversion due to safety or not, and that would be established in flight before the decision is made by the pilot to divert.
That is pretty much what happened to me (I think it came to a little over £26 for landing and 1 overnight).
G