The challenge with the BOH and SOU catchments is the proximity to LHR and LGW. Those living north of SOU have more choice from LHR, and typically less than an hours drive. Those living east of SOU have the same position with LGW , although a little longer journey.
SOU survives mostly by virtue of those resident in the city of SOU and to its southwest and a few sensible travelers from Winchester and Portsmouth who value the ease of use of a local airport.
In other words SOU has the same catchment as BOH! It's rich, it's the largest non industrial population in Europe apparently, and feeds a great deal of traffic to LHR and LGW.
And that is the challenge for any operator at BOH and SOU.
I agree with Cloud, Flybe will stay at SOU, there is no reason to move so far as I can see.