The days of Cathay, South African, QANTAS, Air Canada and all are not going to come back because each of these airlines can offer a codeshare through a STAR ALLIANCE or ONEWORLD hub without fragmenting the UK market by offering service outside London. Hence Cathay and QANTAS are sold connecting on the BA Shuttle and South African and United sell seats on the BMI route over Heathrow. Why would any legacy airline in the Alliance structure serve Manchester when every effort is being put into making Heathrow work as the Alliances UK gateway.
United aren't interested, Cathay balked, South African couldn't do it, Air India lost money. Look at who's left. Non alliance members and strong connections over a US hub with a US airline. Who's left on the wish list?