Mmm, well I personally think it's best not to turn up too soon on longhauls, there's isn't much contention for locker space and priority boarding in its first few minutes is rarely an uplifting experience.

Longhaul boarding at T5 and T3 typically starts around 35 minutes before departure, give or take 5 minutes or so most of the time, 40 minutes for bus gate A10. The gates cannot close before 20 minutes from departure, and often boarding continues until 10 minutes (you can't rely on that and definitely not at A10). So you can safely arrive 21 minutes before departure time, and that includes A10 though they may be looking a bit tetchy if you swan in at that time. LGW longhauls start around 45 minutes beforehand, LCY about 20 minutes.