A 32 year old pensioner writes...
I'm a big fan of the daytime JFK-LHR flight (AA142). I don't sleep well on planes and would rather spend a day reading, working, or watching movies than a day forcing myself to stay awake once I get home as I didn't sleep on the flight the night before.
An extra bonus is that in my experience this flight is often empty. For example last time I took it Y was only 25% occupied - so many people had the 5 middle seats to themselves and could move the armrests up to sleep.
Of course it depends on where you need to get to from LHR: I'm only a 20 minute cab journey away.
Oh, and as for them being 'pensioners' flights -- I'm 32 and didn't feel particularly young last time I took this.