There is a brilliant firm in Blackpool called Bourne Travel who are cruise specialists. They will talk to you over the phone and keep you updated on everything. They are brilliant. My little girl and I are going to New York in October and coming back on the Queen Mary. I think the Queen Mary does New York from Southampton and then on to Fort Lauderdale hun.
If you go on to Yell.Com and type in Bourne Travel Blackpool you will get thier number.
Try Royal Caribbean or Celebrity Cruises. They will have their ships going back to Florida from Sep - Dec each year.
Best cruise t/a is Gills Cruise Centre in Cardiff or Hops Travel
Most major cruise lines have ships in Europe through the summer that reposition back to the Carribean for winter. Go to http://www.cruisecritic.com/
and use the search facilty to find the cruises. These will in the main be in late Autumn.