SEN/MAN would seem to me like the most random of routes, BA axed LGW/MAN, Little Red LHR/MAN, easyJet LTN/LPL as a reasonably comparable route, eastern and BA Express STN/MAN and now Stobart see this as a viable route...
...are there no other points across water in near Europe that could work instead? E.g. Ostend etc.?