Don't forget
Flybe have many years of operations at SOU. As a rule, they know exactly who buys their flights and where those passengers live. Flybe has years worth of data to demonstrate what the real trends are.
I have said on the SOU thread that the potential for some Flybe operations from BOH has merit for that reason. It is also because most of the pax living in the one hour drive catchment area of BOH are that much further in time and distance away from the competing flights available from LHR and LGW.
That said the CI and I think most of the smaller regional France routes in particular will always be much larger at SOU because of the fast rail link to London.
FF