MAN codeshares
Maybe I'm being cynical but Southend-Manchester doesn't strike me as an obvious codeshare route. From Manchester the obvious onward connections are to either the USA (against all the competition from Heathrow and Gatwick) or to Ireland (against Ryanair from Stansted). In the reverse direction it would be up against all the existing routes to major European airports from Easyjet, Jet2, and others. Yes you can set up lots of codeshare agreements, but if passengers find they take longer or cost more than a non-stop flight, they tend to vote with their wallets.
So which routes would be obvious codeshares to combine with SEN-MAN ?