The additional 20 minutes by train from SEN should be more than made up by the swift journey from plane to train at SEN compared with LGW, so the overall time will be similar. Take into account the ease of air traffic access to SEN from the east and it will be highly competitive for flights from Germany, Netherlands and eastern Europe.
Flights from SEN to Carlisle will no doubt serve Stobart's own business interests, but I would use them rather than the seven hour slog by motorway to get to Keswick which I have to do every few months now - not sure if there are enough other Lake District freaks among SE Essex's half a million population to support such flights though!