As everyone knows Motorway services are a rip off.
However something I found our a while ago is, when I show my AA card I get a 20% discount. The discount is in all areas, shop, cafe, Burger King etc.
I often stop for a Burger King and it brings it down to a more "real" price.
The same applies if you have a RAC, MOD pass or Police (force not group hehe) card.