As stated above the guys on the ground dont know what to look for when looking at insurance documents and the MoD reqire the wording to be exact so a copy of your insurance has to go off to the MoD ( Strike command at High Wycombe administer this ) and a cirtificate is issued for the life of your insurance policy.
The cirtificate number is usualy asked for when ever requesting to use a military airfield
I think that the system was put in place with the aim of keeping civil aircraft out of military airfields wile telling MP's that the RAF are trying to recover money for the tax payer, it is a bit of a pain to get the paperwork done but I have found the RAF very helpfull to work with as long as you understand there funny little habits like QFE !.