Coming in a bit late on this thread, but have much experience of bringing mince pies back to US before Christmas - personally I always go for Sainsbury's Taste The Difference, bought before getting to LHR. I have to fit them in my hand luggage as only a totally rigid suitcase seems to prevent pastry damage in hold/baggage handling. I have taken to removing and declaring to T5 security staff all my mince pies, and other foodstuffs that might appear as semi-liquid, after one episode where bags of Jelly Babies packed next to my coiled laptop cable arose interest. Putting boxes of Mince Pies on the security belt, can, of course, provoke humour even at T5, though usually along the lines of "you'll have to leave those behind sir, we'll find a good home for them", but well worth it, as once visible they always get through. Chances of dreaded secondary bag search are also much reduced.