I seem to remember a comment back along that there are no dispensations for flights that were scheduled to land at LHR being late, you either use a "late home" credit and a telling off from your mum, or you replan the flight.
Emergency diverts that were not due to land at LHR are I believe a different situation, whether there is a polite conversation whether STN or LGW could be entertained rather than waking up West London, but if a commander wants to emergency land at LHR and broadly there's no physical impediment then broadly LHR is permitted. But if I have misremembered hopefully
Heathrow Tower will be along presently to correct my memory.
We are so fortunate on the BA board to have industry insiders who are prepared to share their time and knowledge with us.