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.
Would a bit of marketing from the West Midlands, to both passengers and airlines, depress the price that Ferrovial et al are expecting to get for Gatwick?