Are you aware that WAW (which is an airport located almost in the downtown area) has curfew from 11PM to 6AM? (They only made some exceptions for LOT and Wizz.)

If your BA flight is delayed by 31 minutes, they might be forced to cancel it. I know that each year airlines serving WAW are given a couple of curfew exceptions, but once these are used there is no mercy. I personally try not to book evening flights to WAW on airline other than LO.