You're right, impossible in GDN. Bus to the non-Schengen far end of the arrivals hall, immigration, baggage hall, customs and exit, turn right to departures where no doubt BA will not pay for fast track, back through security, through the duty free maze, double back on yourself, long walk to the far end of the terminal for passport control, keep going to the far non-Schengen gate, by which time the aircraft will be half way back to LHR.