Chaps, I'm very much with Chilli here.
I often pop into a local fighter base. On the first couple of times (in different aircraft) , the bods in the tower would phone the squadron where I'd parked my "jet" and ask about Crown Indemnity.
On the first occasion it took a bit of phoning around and maneouvering before I could categoricaly quote the waiver number. That number, was my policy number. Nowadays I just carry a photocopy of the policy, which states "Crown Indemnity £xM" and Robert is your mother's brother.
In general, I fear it's safe to say that, they (scope-dopes in the twr) have no more of a clue what to look for than you!