Birmingham may well be at the centre of the UK in overall population number terms.
However, the centre of the people who actually buy air tickets is very firmly based in London and the South East.
And the centre of where the people who are coming INBOUND to the UK, which is 50% of total demand, want to go to is most certainly there. How many of those from, say, Russia, with its multiple daily flights into the UK, want to go to Birmingham ? Zilch.
Maybe Birmingham can go off instead and come up with huge, taxpayer-funded plans to become the new centre of the UK sea fishing industry.