As others have alluded to, it depends on the metal. The delta between J/F on the 77W is miniscule - unless you are taking a daytime flight and don't want to interrupt work while you eat - I do actually adore being able to swivel the seat to face the window and separate desk area in F on the 77W. If an overnight, then J all the way. It will be more of an upgrade from J to F on BA metal unless you're in their newest cabin.

If in J as EXP, then the CX First Lounge in T3 is the best you can do at LHR. It's a very good lounge. If you're in T5, the Galleries lounges would be fine if they weren't always so packed.