Example: A colleague of mine has just booked a trip to Vietnam. Tried to get on SQ ex-MAN, no seats available. EK and EY wanted c.£1500 ex-MAN, and so she ended up on Thai out of LHR for less than £1000. That sort of saving blows all the convenience / loyality to local airport sentiments out of the water. So-called 'low revenue' MAN is top-drawer for the punter. I'm sure the airline economists amongst PPRUNErs will explain.
A perfect example of supply and demand. LHR has dozens of routes going east via dozens of hubs or non-stop. Hence far more supply. It's also the reason why the fares are often similar or less from LHR - more supply from more carriers = more competition = more passenger friendly fares.
Often EK will have higher fares from MAN than LHR.