Bose-x
You may well be correct. I've only rented from one club, and only been a member of one group (my current group). I never went cross chennel with a club aircraft, but I definately remember other members filling out drawback forms. My group only reimburse what the receipt says, so I always go for the expensive stuff, and have the hassle of claiming back the money from C&E.
I agree it's better to get the duty free stuff if you can get reimbursed at the 'normal' price.