I actually think the stars may align to the benefit of BRS in this case.As far as BRS is concerned they would have to get their current 10 mppa planning cap raised to take advantage. It would be ironic if the LHR third runway failed to progress because of planning decisions and BRS expansion was also blocked because of the planners.
The green lobby will very likely scupper the LHR third runway. With the next Climate Conference in Glasgow in 2 years time, the UK would need to be seen to be leading the way, so at the very least the issue will be kicked into the long grass. Remember Boris has in the past been no fan of expanding LHR, and now his constituency is rather too close for comfort!
However, as a result of the referendum and last month's election, it seems inevitable that the centre of gravity for domestic economic investment will escape the confines of the M25 to some extent. The focus of UK trade policy also seems likely to veer westwards for similar reasons.
Therefore growth in air travel outside LHR will still be needed and airports like BRS could (comparatively) quietly add a couple of TATL flights 'under the radar' without the hullabaloo and gnashing of teeth that would greet further LHR expansion. MAN and NCL would also likely benefit, but the 'levelling up' I keep hearing from government ministers will take longer and cost more in the north/midlands than it will in the already relatively productive BRS catchment.
Some tactical investment will be needed to fill the vacuum caused by any gravitational pull away from the south east, and the south west is an obvious candidate to support that.