Lots of great advice here. I'd definitely emphasize the idea of "pace yourself". It can be tempting to chug 4-5 glasses in the F lounge, then, by the time you board, you're already feeling half-cut, and more champagne or other drinks doesn't feel so appealing. So I'd say, try a bit of everything, enjoy the lounge, enjoy the first 2.5 hours of in-flight service, then enjoy the bed. For me, one of the treats of F on a night flight is a really comfortable bed to sleep on and arriving rested. BA has some decent digestifs, too, which I like to ask for as they prepare the bed, so I can enjoy a sip of tequila then drift off into Morpheus's arms at 40k ft.