How fine should I cut it at BGI

So returning to England today after 3 weeks in BIM
Enjoyable trip mostly apart from the nightmare in laws
Travelling on own as rest of family are coming back tommorow. Flights at 17.45 and gate closes at 17.15
Nothing much to do at BGI so what do you think the latest I can leave it to get to the airport to maximise beach time
Also as a side note, does anybody know if there is Premuim seating on the Thompson flights (Dreamliner) that go to Barbados (although next trip likely to be BA as totally rinsed my Virgin miles now,better to spend than get them devalued)
The last week we have been South Coast so not too far to the Airport.

Enjoyable trip mostly apart from the nightmare in laws
Travelling on own as rest of family are coming back tommorow. Flights at 17.45 and gate closes at 17.15
Nothing much to do at BGI so what do you think the latest I can leave it to get to the airport to maximise beach time

Also as a side note, does anybody know if there is Premuim seating on the Thompson flights (Dreamliner) that go to Barbados (although next trip likely to be BA as totally rinsed my Virgin miles now,better to spend than get them devalued)
The last week we have been South Coast so not too far to the Airport.