If you aren't on a sleeper service, I would definitely recommend eating in town or in the airport before entering the lounge. The nibbles on offer would hardly satisfy an anorexic gerbil.

My usual plan for an evening flight from JFK or BOS is to eat, self-medicate with alcohol in the lounge then fall asleep on the plane as soon as possible to get maximum rest on the short UK-bound flight. If you stay awake for the onboard meal service you just don't get enough shuteye. I always sleep through "breakfast" too.