I can not honestly see how a Carlisle-Southend service would work. Given that the service would be mainly used by business travelers then a comparison needs to be taken with existing rail services. Virgin offer a first service of the day 0543 arr 0904 for £150 walk up single. For the air to compare it would have to have an airport arrival at 0545, off stand at 0615, on stand at 0730ish, transfer to station and depart at 0800 for an hours journey to London.
To be competitive on cost and time it would need a walk up fare of £140 single.
A few chaps I know do the sensible thing, book the sleeper from Carlisle to London the night before, leave their cabin after breakfast and use the shower facilities to change before leaving Euston about 0800 for whatever they have to do. First Class fare about the same as Virgin.
If it was Carlisle-Luton (tapping into Wizzair/EasyJet) to Stansted or even Gatwick it could work as they are hub airports.
IMHO Stobarts just want the land and the HGV terminal. Given that, then air operations would wither on the vine in less than a few years.