If I was easyJet I would want to defend my LGW operation and consolidate there. SOU is a costly airport to operate from (having no other sources of commercial revenue) BUT easyJet Holidays do seem to compete well with Jet2 Holidays at other bases. Look at BRS for example.
Jet2 does not operate from LGW so easyJet has a strategic advantage there. Is there any advantage to easyJet purposely loosing £m's by basing aircraft at SOU in order to defend LGW? Crazier things have been done so perhaps one should never say never.
Replicating what they do at SEN at SOU? They operate from SEN because they do not serve STN in any significant way, so we can't really draw comparisons there.