Don't know if it still happens as now most of BA are in T5 but their early morning long haul arrivals at LHR Terminal 4 sometimes had to shut down and be towed onto "S" or "V" stands before 0700hrs local due to local planning conditions (noise).
Interesting question regarding collision - is the aircraft legally deemed to be "in flight" with engines shut down being towed onto stand with pax on board.