Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
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Posts: 63,846
You can check-in any time on the day of travel, it's normally 3 hours before but anyone with lounge access can check-in on the day anytime. Check-in opening varies a bit by day of the week and time of the year, but you can use 03:30 as a guide there, though the lounges won't be open at that unGodly hour.
Those without status can only check in a maximum of three hours before, though there's no need to be at LGW that early.
There is, however, one LGW special option, also shared with EDI: you can check in the day before too, between 16:00 hrs and 20:00 hrs. More useful for those staying in the hotels before an early flight, but nevertheless it is open to all. I wouldn't leave it to the last stroke of 20:00 hrs, however, since occasionally the last flight out of LGW means they can close up a bit sooner