I think that TAY26H from EGPH (Edinburgh) to EBLG (Liege) is a usual flight on the Electra most evenings. I work night shifts just inside the M25 and have heard it a few times, hence why I looked it up.
It routes N0300F190 TLA N57 POL R101 STAFA L10 HON/N0314F180 L186 BIG/N0300F190 W70 KOK, departing EGPH at 20:55 and so is normally over London (BIG = Biggin Hill VOR) at approx 22:05 at 18,000ft. Does this sound like the one you're hearing?