I used to work in travel in the 80s when Britsh Midland flew 5 DC9s a day from LHR to LPL. Our travel office put quite a bit of business through them, but the route had stiff competition from BA’s MAN shuttle which was more frequent, plus at the time Liverpool as a City was not doing too well. The train has improved of recent years and its really a better option city centre to city centre, but what is still a real problem is using the train and getting to LHR.
In my opinion regional air links severed from LHR, just divert passenger to easier hubs like Amsterdam.
I flew on BMI from MAN to LHR on the last flight of the day on a Friday about 3 weeks back, sat on a very comfortable A319 with about 20 on board.