Mince pies definitely fine for security at NCL, EDI and LHR. Done repeatedly.

But I'm very surprised you are asking, there is now a Fortnum & Mason under South Galleries. And they have multiple sorts of pies in various sizes and packaging. Unless, of course, the inner Scotsman was adding length to your pockets.

There's also a very prominent Walkers' deal at WH Smith between North and South Security. I can't remember the exact details but it was something like 3 boxes of 6 for £10 and I remember walking away thinking that maybe my inner Scotsman was awakening at that price.