LHR requires you to go through transit security (unlike FRA, MUC, or ZRH) and there can be some long-ish walks in the terminal that *A uses. That said, transfer security has been lightning quick every time I've used it (unlike other terminals at LHR, probably due to the low volume of connecting *A passengers at a non-*A hub), and I don't mind the walks before or after a long-haul flight.