I can't quite understand why people are so sniffy about SEN as an airport serving London.
The facts speak for themselves in that an integrated rail station will offer journey times from SEN to London which will be as fast as from the other London area airports bearing in mind a probable quicker transit through immigration and baggage reclaim onto the platform. A few years ago LTN would not have been considered as 'London' and STN was just a strip of concrete in the Essex countryside far from the capital. It is the actual journey times which matter, not some old fashioned image of the town in which the airport is located.
The launch of a CAX-SEN service could provide a through service to AMS, as Phileas Fogg suggests. At a stroke you then have a Carlisle to London day return service and a Carlisle to Amsterdam a.m. arrival with an early evening Amsterdam to Carlisle return service. Add to that the fact it also gives you a twice daily SEN-AMS-SEN and the figures start to make sense. The icing on the cake would be to have AF/KL operate the route and you have full through-booking capability to worldwide destinations.
Stobarts certainly have full details of a workable schedule so it's down to them to convince an appropriate operator of its merits.