EasyJet-4
It's a leisurely one minute stroll from the station to the terminal. I suppose it just might be possible - it's very difficult I know - for the rail company to put on earlier and later trains....? And what's all this about wind and rain? This is the Essex Riviera you're talking about - the rain this week was the first for four months.
STN won't be affected too greatly by the growth of SEN, but, even if it is, just look back one last time and see it as returning the flights and jobs which moved from SEN to STN when Channel AW moved in 1970.
Much of the passenger numbers will come from Europe using SEN as a convenient London terminal, but don't disregard the local catchment. There's a quarter of a million people within a short distance of SEN and they will be a useful addition to the load factors. STN remains in a rather less densely populated area of the county.
Good luck to SEN and EZY - they will be a highly successful partnership, I'm sure.