With apologies for this being a bit of a repeat posting, but here are the shop prices (or LGW airport prices to be more precise) for some M&S items:
The uplift - if any - seems quite modest, maybe 50 pence at most. At this stage of the game at any rate.
The uplift on drinks, on the other hand, seems higher. I'm an expert only on sparkling water at airports, and 500 mls is available for under £1 (Pret, Starbucks) and around £1.35 at WH Smith, though it can be higher in some branches. So £1.80 seems a bit stiff to me.