punters don't actually want to go to/from an airport in the West Sussex countryside
With all due respect, I think this view is somewhat outdated. Without wishing to get onto the merits of LGW too much here, you are 30 minutes from Central London and have a increasingly wide range of international connections available. However, obviously codeshares would be useful and BA/Brymon was always a good way to ensure this in the past.
Also, the figures you quote regarding bums on seats are somewhat dated. We know that since the 1970s/1980s the commercial aviation industry has grown hugely - fares have come down and traveller numbers have gone up. This can only be reflected by more patrons of any PLH - LON service.
I would be confident in a SEN service working out on ATR 42s, Dash 8-300s or Jetstream 31s/41s. Door-to-door from Plymouth would be 45 minutes check-in, 45 minute flight, 10 minutes to pick up luggage and then 52 minutes to Liverpool Street - total is 2h32m, compared to an average of 3h20m by train. Given that train fares aren't cheap (to say the least), this could probably work pretty well.