LHR doesn't have a straight curfew like some airports, but as the evening wears on it does become both difficult and expensive for BA to run a late service. There may not be the ground staff, engineers, crew on board run out of hours and at some point the slide to cancellation becomes difficult to stop.

As things stand you have a calculated take off time of 23:37 and an estimated take off time of 23:57 and I'd suggest it could look like a pushback from C53 at about 23:20 or 23:30. They are on a target pushback of 23:15 hrs, but since they haven't started boarding that seems unlikely.

Just typed all of that up: it's now cancelled! G-ZBKB.