I rolled my eyes on reading the title and laughed out loud about the fuel comment!
Let me deconstruct your post.
If you give, for example, 110 economy passengesr £5 and 15 business passengers (on an A319) £10 you're paying out £700 per flight.
It does not cost that much to cater through GG. A tabloid threw some figures around during the strike but it was half that figure at least. So let's assume you're paying an extra £350 a flight.
All Day Deli boxes are hardly heavy and if you remember that passengers are going to bring on their own food which might be heavier I can't see how you're going to save on weight and over £350s worth of fuel!
Edit: if shorthaul BA Y went low-cost that would surprise me, but that is a different matter.