There is a
proposed legislation that would allow UK airlines to operate flights into the EU for limited period of time after UK's withdrawal
on the condition that number of frequencies operated by the airline to each member state will not exceed the amount of frequencies they operated in previous timetables. This means BA would be able to run the same amount of flights into the Greece as they do now (
if the legislation is approved), but they would be unable to increase frequencies.
Problem is that some carriers already planned to increase frequencies for next timetable and are already selling additional flights. These carriers may be eventually forced to cancel some flights. Whether this concerns BA's flights to Greece or Romania is unknown to me as I don't follow their schedules in such detail.