Try reading the history of the route applications between 1945 and 1992 backed by the airport but fought by and successfull defeated by BEA, BOAC and BA. Read how successful routes by SABENA and Aer Lingus, were stopped by the UK government at the behest of the state airlines.
Same at PIK when BOAC forced out KLM and SAS DC8 stoppers, now of course that would have stopped anyway but all locals remember is BOAC demanding a monoploy, getting one then er....leaving. The thing to bare in mind is that Emirates, Qatar and Etihad are succesful instruments of national policy, much loved by their governments and de facto flag carriers.
Aer Lingus, BA, Swiss, Lufthansa, KLM and Air France (OK not that last one) are having to survive in a less cushioned environment shall we say.
To the specific point, the SE is a victim of it's own success. So in my mind there is an opportunity to have a lot more MAN-long haul so that no one in their right mind would choose LHR over a local MAN option (cost permitting!)(. In order to do that, one has to make a commercial business case that will get the nod from an airline looking for a good ROI. Now United are not offering a strong MAN-LHR-US offering, nor are Delta, only BA/AA are strong. So to my mind, why have the non OneWorld US legacies not been giving MAN the offering it arguably deserves. Delta with a single B757? United still with the B757? Genuine question, what are their frequent fliers using instead of BA/AA? Are the Delta guys doing KLM over AMS to connect with the Delta hub? What are the United FFs doing?