JSCL,
I've never been in airport commercial however much of a problem around UK is airports have taken over ex military airbases, expanses of land, tarmac etc, all of which need to be paid for, maintained, secured etc.
One airport, that springs to mind as being different, is Dundee, ideally an airport runway should be longer than Dundee but I'm talking about the size of the airport grounds, it's small and, in that respect, cost effective, get such a ground space close to Plymouth then that would be a good place to start.
An airport needs it's 'bread & butter' such as on site flying school, perhaps an exec aircraft centre, a maintenance facility, as far as airline routes are concerned well, I'm guestimating, PLH warrants a 30 (ish) seater aircraft to operate a business route morning and evening, LHR slots have gone, LGW doesn't offer enough connections, so perhaps MAN, certainly during the summer a middle of the day Channel Islands, a middle of the day Ireland service, think Ireland would work year round, perhaps CI also, that's one airliner pretty much occupied and an airport taking in the revenue from car parking, cafateria sales etc, and of course landing/navigation and aircraft parking fees, never mind the rent for the home based airline's hangarage or whatever.