If the Gatwick locals don't want a second runway (and the Heathrow locals don't want a third), then the train journey to BHX is only a little longer (less than 20 minutes from parts of central London) and likely to be further reduced over the next few months.
Yes let's all fly from Birmingham. Can't think why I never thought of that. Genius. I guess if you take the northernmost part of the M25 then it might be 20 minutes nearer to BHX than LGW, but that's quite seriously clutching at straws.