really not sure I would bother with Galleries F - just take the little door to CCR out of security and hang out there - at most step out for a drink on the terrace in GF
you have a 12 hour flight with champagne / wine / lunch to enjoy as well: I wouldn't get to LHR before 13.00 for a 16.00 flight to be honest, super long day - get a lie-in at home and enjoy the F experience in the air