While the DTV situation comes to mind, Carlisle has the benefit of being much further from a competing airport than DTV is from Newcastle. For locals to reach NCL, MAN or GLA there is a nontrivial cost in both time and money (eg train fare, taxi or long term car park). This could allow the likes of Thomson to charge a bit more than at MAN, effectively selling convenience and overall money saving at a higher airfare. Of course it would likely be a 1x wekly flight on Tues or Wed - can what is a rural area with low population density fill bums on seats ?
PMI / ALC at Carlisle is certainly an innovative idea showing imaginative bluesky thinking and should perhaps be encouraged, even if it's still a good few years from coming into reality.