Flying ex-LHR in F you will have access to the Concorde Room, which is very nice indeed.
The seat is comfier, privacy better, food better (ex-LHR is not usually too bad), service usually better.
If you have the miles, go for it. But if you are trying to do a dummy miles booking, and can't see availability in F for your flight, then you're out of luck (for the time being). Keep checking though, as seats may open up.