It would be hugely expensive to maintain a batch of spare stands to cater for the relatively few days per year (typically around 15) when things go pear-shaped.
Same applies to spare aircraft, spare runways, spare anything in fact ...
Although many other airports and carriers seem to manage this quite OK.
Take little Helsinki. One-fifth the number of pax that Heathrow handles. But they managed to justify opening a third main runway a few years ago, and also have a large remote parking stand. I've hardly been sent there, but on the couple of occasions when I have because the jetways couldn't handle us, all the ground support equipment and enough passenger buses have been sat waiting for us.
Someone at BA must surely notice that at their overseas destinations, overcrowded or not, the ground crew are invariably in place when the aircraft arrives, whether late, on time, or early, whereas at Heathrow in my experience it's a 50% chance you have to wait for people to turn up. "Waiting for a stand", along with "we're just a little early", or (more commonly), "we're just a little late" is a hollow call when the stand can be seen to be empty, along with others, and you are blocking those around waiting for the Stand Guidance to be turned on.