Until the "Londoncentricity" of the UK is reversed I'm afraid that all other cities, and their airports, will be under used.
This is unrealistic. It's not that BHX is close to the capital of the UK, it's too close to one of the
world's top cities. Comparisons with Germany aren't entirely spot on as Germany has no equivalent of London. Take a breath, no, it does not. Paris comes closer but even London is now the 6th largest French city on Earth, telling on internationalism.
London is a magnet, I live there, I am from the "regions". It's just no comparison, and when you go down the social engineering path, you end up doing silly expensive and impractical things like moving the BBC to Salford Quays and still having to duplicate the infrastructure back in London! Still lets not be happy about our own capital city being such a force in the world, ' cos it's making our good old regions look bad.....
Seriously London-
Birmingham? Don't be ludicrous. We can't even make a serious stab at London Gatwick and Stansted as serious airports with legacy carriers and connections, thought GIP are making a great effort and showing progress at Gatters.
Does any other country in Europe have quite so many airports with long haul as we do? Look at Emirates, they serve GLA, NCL, MAN, BHX, LHR and LGW in the UK and DUB across the sea, that's SEVEN airports in the market. This does not happen in any other European country. (They only serve ~33 airports in Europe btw)
On a UK strategic level, we're not underserved, we're overserved. Airports left, right and centre. Two in Belfast, two for Glasgow and both fighting EDI, MAN vs LPL vs LBA, NCL vs MME, EMA vs BHX (no longer vs CVT), DSA thrown in the mix. London now has LHR / LGW / LCY, STN / LTN and if that wasn't enough we now have SEN. None of this is rationally planned or strategic. All politics is local and no city must be without an airport in what is frankly becoming a willie waving competition.
This does show signs of returning to some rationality as MAN begins to grow again as the shine comes off LPL and DSA retracts, BHD loses WW and CVT is closed to commercial airliners. Even PIK is in retreat as the Ryanair bubble floats overseas where the subsidies are higher.