Five of the six destinations have no competing London link and Antwerp just 200seats per day to LCY. So if there is demand for London, people will use the SEN flights. With the exception of ANR, they really have no alternative - no other carrier and no nearby airport with a London link.
I wish Stobart the best of luck. The big question really is how the strategy to serve regional airports with no London link leaves room for future expansion. The number of regional airports that are too small to attract a larger operation but big enough to sustain a daily ATR72 flight to London - and near enough to be served by a turboprop - is rather limited in my book.