I cannot see Jet2 coming to make a base at SEN (as much as folk and spotters may like it) because their newish base up the road at STN is doing rather well -
However the opportunity is there for them if they were to take the risk (but Jet2 tend not to take risks, and SEN's operational hours are too restrictive to an IT airline).
They won't go to LGW - Jet2 like to self-handle and I'm not sure they would be able to at LGW, plus the slots issue and the high costs.