If you are checking bags, the earliest you can hand them over is the day before - with some limitations: https://www.britishairways.com/en-gb...ernightbagdrop

"Overnight Bag Drop is available from 16:00-22:00 for flights departing the next day before 10:00 from London Heathrow Terminal 3 and Terminal 5. It is also available from 18:00-21:00 for flights departing the next day before 13:00 from London Gatwick. "