Despite all written above it's a uniquely British problem. It used to afflict just Heathrow but has now spread to other airports in the country.
We all experience wherever else we fly to that ground crew are somehow normally awaiting our arrival. In the UK, not so. 15 minutes ahead of schedule, or behind, and that's an excuse. If you chance to be right on the minute, then it's No Stand Available. In my experience it's quite rare to arrive on BA at LHR and all the necessary resources are in place.
I fly BA in and out of far northern, snow-bound airports, where they operate in and out without difficulty. De-icing somehow turns up on time. Runway clearance is maintained. Their ops people must see this. Yet at Heathrow a fraction of these conditions causes everything to fall apart.
The recent Ryanair issues have caused gross difficulties with returning flights delayed to after midnight, and no ground crews available. As Ryanair generally operate out-and-back, the delay is typically predictable the moment it sets off, because the outstations seem to be able to turn the late arrival round in the normal time. But back in the UK - "all gone 'ome, mate".