If my memory is correct, BA announced their withdrawal from the LHR route after planning permission had been granted for the extension of the runway, or at any rate whilst they were still operating from the airport. Had BA had the will, they could have continued the service during the construction period by either scraping a few more years out of the Viscounts, or by using jet equipment with restricted loads (EI used B11s and 737s on the old length runway for years).
I did see a comment in 'The Wharfedale' a few weeks ago which said that reinstating the Heathrow route was one of the airport's priorities.