This may be a bit simplistic but if and when the runway is extended and BHX can smooth the international arrivals procedure out, then with three trains an hour to central London it could be an attractive proposition to arrive there.
I can not be beyond the wit for a deal to be struck with Beardies bunch to provide cut price first class travel from B'ham International to Euston. If it could be done then arrive, immigration, transit to station, hours journey, fed and watered, London in maybe a couple of hours or so from wheels down. It could compare well to busy times at LHR & LGW.
Or is that just a load of bollix?