That's really interesting CCGE29. Certainly it could make sense, it's easier to fly SEN-MAN to connect than take a train and cheaper probably, but LGW and LHR are not far and as LTNman has pointed out from Dec 2019 Crossrail will offer a simple (if not that swift) connection to our main hub which will also be relatively cheap.
I think the pros and cons of SEN-MAN are balanced and there could be the problem. People don't change from the tried and trusted unless there is sufficient incentive to tempt them.