You can bring in a carton plus whatever you buy from duty free aboard the plane; depending on how long I'll be, I usually pick up a carton or two on the plane or in the duty-free shop. Never had a problem heading into the UK or anywhere in Europe for that matter. Don't put any in your checked baggage -- they might disappear, but there are no restrictions AFAIK in terms of putting them in checked baggage or in a carry-on. FWIW, they're usually cheaper in duty-free than here in CA.