Maybe 50% of the UK resident population find Manchester more convenient than London but the percentage is diminishing every year as the South-East population increases and that in the North decreases.
However the number of overseas air trips generated per head in the South East is at least double that in the North, and the number of premium class trips generated per head in the South East is maybe 5 times or more that in the North, unless you have to give the premium tickets away to fill the seats. As carriers like Continental or Delta will tell you.
Furthermore that is only outgoing passengers. For inbound from overseas the number who want London, not Manchester is up at 80% plus of the total. You don't find this in some other countries (eg Germany) where demand is spread, but do find it in say Russia, where 90% of overseas demand is to/from Moscow.
Facts of life. Get used to them. Unlike the old Soviet Union we don't have central planners telling the airlines where to go in accordance with political posturing, we leave it to the travellers to decide where they want to go. And they have decided.