NickBarnes beat me to it with his post but his post is spot on.
Passenger numbers are price sensitive so Easyjet should never have a problem filling seats as the aircraft starts to fill with passengers buying discounted tickets but I would have thought that tickets prices for seat 50 and seat 70 would be substantially lower than Stobart could offer them for if it followed the Easyjet pricing model.
Stobart might well have a different model for pricing tickets with higher low priced tickets and cheaper maximum priced tickets. If the airline gets the balance right then there is no reason why domestic routes could not succeed. Also a prop aircraft is best suited for short duration routes so will have a lower fuel burn which all helps to keep ticket prices down.
BUT London passengers have a big choice of airports to fly from and while most would be checking out Easyjet, BA and Ryanair's websites how many would be looking at FlyBe's website? Even if they did most passengers would vote with their wallets so Stobart would still have to aline ticket prices close to the likes of Easyjet which would hurt margins.