Mathers,
From personal experience I can tell you that BHX's catchment area spreads as far east as 'Rushden & Diamonds', as far north as mid point to MAN, covers mid Wales also and mid point to BRS and, I guess, LHR.
When I was living in Rushden and then Telford and then Coalbrookdale the likes of KLM, Swiss, Lufty, Frog Air and SAS were attracting my business from/to Brum ... go figure what chance they'd have of attracting my business from/to CWL?
Brum, as far as aviation is concerned anyway, is England & Wales third city and that's not about to change anytime soon.
And please don't think that I'm anti CWL, I worked at CWL in the 80/90's and I only sold my CF62 postcode house last year.