The abbreviated "supper" service is fine, IMHO, for these late departures to LHR (although my experience is only from JFK). I remember a number of years ago flying F JFK-LHR and like 75% of the cabin wanted the express meal so they could go to sleep. So the FAs had to do all those first and then we were halfway to London by the time they finished with the rest of us.
In any event, there's plenty to eat in the Flagship Lounge before departure, so I wouldn't worry about the abbreviated service. And, IIRC, there is still an appetizer as well as a soup (you can start with both), followed by "entree dumped on lettuce."