What they're building is still on the hopeful side of realistic. The regional flying ship has long since sailed, and we're seeing that all over Europe. If this airport can succeed then the following airports would be small but thriving bases of commercial passenger operations:
Blackpool, Cambridge, Gloucester, Manston, Oxford, Plymouth, Sheffield, Shoreham, Swansea.
They would have flights to Amsterdam, Belfast and / or Dublin, and London or Edinburgh (depending on distance).
That they aren't is a little bit sad :-( But the fact there are so many reasons this can't be the case means Carlisle has a struggle ahead.