One thing could make a HUGE difference to Birmingham's attractiveness as an airport for Londoners.
When you're going TO BHX you can book your train ticket months in advance for the cheapest price, but coming back you have no idea exactly when you will be through arrivals, so have to either book a couple of cheap tickets an hour or so apart, book an any train ticket at greater expense, leave a huge safety margin, or take a chance that you'll be hearing that Ryanair fanfare.
Virgin Trains could work with BHX to offer some sort of product whereby if your incoming flight was delayed (or arrived early) your advance purchase train ticket would be valid on an alternative train and maybe charge a £1 or £2 premium for this insurance, (call it what you will).
Until this happens flying into BHX will never be attractive to many Londoners who might choose BHX if the price was right.