EZY/RYR flights out of SEN is (was) mainly operated by based crews. bad bear was pointing out the difficulties in having an operation at SOU based mainly on 'W' flights flown by non-based crews.
Basically any airline operating a "W" pattern through Southampton would need to ensure that the plane and the crew are certified as above. This means either all of the planes and crew need training/ certifying or only the few that operate to Southampton and risk disruption if there is a crew or plane change due to operational reasons.