I'm surprised at the negativity.
The cost of £200.00 is actually not excessive. I think you will find the absolutely useless Virgin West Coast more expensive even for their Cattle Class of a morning, let alone first. (Actually its thanks to Virgin for the large increase in domestic flights from the North West. they have done an excellent job to promote air travel).
For a lot of people in that area, it is time consuming enough to get to Carlisle, if the option of a flight was available from their then it would appeal to a lot of people. The demand is certainly there, it just hasn't been tapped.
At the moment, they will use LPL for LCC, GLA and MAN for long haul (most also appear to use London by utilising the train if not at peak hours) and PIK and NCL for other options. Therefore the market is quite fractured at the moment.
I can't speak on the operating costs of aircraft, and the best London airport to utilise would probably be LCY (perhaps an option for VLM?). It would also benefit from some interlining.
Other potential options for Carlisle could possibly be a seasonal service to IOM, DUB, BFS and Jersey but I doubt if it would be feasible for much else.