I dare say that a Carlisle to London air service may well be a poular alternative to the rail service, but it doesn't matter if it was a Dash 8 Q400 which was 75% full every day, it still wouldn't be economically viable.
The cost of operating into Heathrow, Gatwick or London City is collosall, even if the operator was able to offer interline facilities to attract onward travelling passengers, the profit on tickets from Carlisle to London would be very small and not economically viable.
A service to Stansted or Luton is still outside the realms of low cost travel, a service to Southend would have to be substantially subsidised by both airports for any operator to even discuss it, never mind operate it and would still leave the passenger with an expensive onward rail journey into London.
The protaganists for the expansion of Carlisle Airport conveniently forget to write that the Stobart Group debt is in the region of £180m and although continuing operatingprofits make that perfectly managable, any investment and attracting capital for that investment, is going to be on projects that make a return within the first year or so.
There may well be good grounds for spending money at Southend Airport, but at Carlisle, regardless of papers signed etc, it's going to be on the industrial infrastructure, not any significant facet of the airport.