The only scheduled airlines on that route are easyJet (with 'U2' flight codes) and Ryanair (using 'FR').
All airlines flying into Alicante are listed here:
http://www.aena.es/cs...9236&pagename=subHome
but I can't see any likely airlines for your flight.
It could be that your travel operator has used their own flight number (choosing a random code that's not associated with any airline operating out of Stansted), simply because (at the time they started accepting bookings) they'd not confirmed which charter firm they'd be using for the service. If so, it could be any airline.
Chris