Checking Travelex in T5 for a departure in the next week: I see that they advertise as being Landside near Huxleys Restaurant but the LHR website has no knowledge of that restaurant - so it is probably closed. Travelex say that their Airside desk is 'Coming in Spring 2022'. So that might not be there. Either way, all branches close at 18:00. I was not expecting to get to the terminal until 18:00 so might now need to change time or plans. I see BA departures currently up to the 22:00 hour. Obviously those Pax do not want currency services ...
IIRC, Huxleys is/was airside, just beyond security and near the BA lounge and the escalators. Who knows if it is still there!