One reasoning says obey the rules,they don't really want to talk to you if you are not in their controlled airspace,as they've got enough of their own traffic.
They will know within a millimetre where you are.
The other reasoning says even if you're outside their limits they can advise you of heavy traffic,which if you fly near could really spoil your day.
When I've actually required a Matz transit round here ,the USAF have always been most helpful,I've found if you talk slowly,they answer slowly.
Never had reason to call RAF bases but I'm sure they're the same.
