The reality is that in the U.K., IAG is competing with itself. Yes, other hubs and other alliances are available from the UK regions but the main competition is surface transport to Heathrow.
...which is not really convenient if referring to rail. The problem is the lack of a comprehensive domestic network (currently only 8 destinations) to feed longhaul. A series of codeshares could allow the convenience of through-ticketing.
Rail cannot offer that, what rail offers is the inconvenience of travel via London, as no long-distance trains stop at Heathrow.
As always with Heathrow, it comes back to the old chestnut: chronic lack of rwy capacity over many decades.