Hi chicken legs.
I bought my first case in a shop in county mall, although 9 years ago. they offered a small discount.
Second case was bought in LGW itself at the left baggage shop and again offered a discount.
If your on longhaul, the Samsonite shop in 'point Orlando' mall is very cheap and offer crew discount, and what with the $ rate at the mo, its worth waiting till you go on a trip.
To be honest, try anywhere and politely ask for a discount. You will be suprised what that I.D. can get you.
regards..