The only scheduled transatlantic crossings are on Queen Mary 2 (QM2). They are 7 nights in length, and operate Westbound from Southampton to New York, and Eastbound from New York to Southampton. Depending on the schedule you can take QM2 in one direction and fly in the other, although occasionally it is possible to take the QM2 in both directions.
A re-positioning cruise is another good way of spending a number of days at sea, which can be more relaxing that a port intensive itinerary. A lot of ships are based in the Caribbean or Florida during the British winter and in the Mediterranean or the UK in the British Summer. Therefore they need to re-position themselves at the end of each season, which often leads to some advantageous prices for those happy to spend time at sea. These normally take place in March or April to the UK or Mediterranean and in September or October from the UK or Mediterranean.
An example would be to fly from the UK to Barbados early April and board your ship, and cruise to 4 or 5 Caribbean Islands. You would then depart for Southampton, with one stop on the way, perhaps in the Azores or Canary Islands. This makes a great 12 to 14 night holiday, combining just one flight, some great Caribbean Islands, and a number of days at sea to just relax.
(Page updated April 2012)