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P&O do round trips to the Caribbean from Southamption. Normally 22 days
You don't say if you want to travel this year (2008), or next year (2009). Either way the only company I know of that sails direct from Southampton to the Caribbean and returning to Southampton is P&O. Best bet is to check on the P&O website pocruises.com for availability. I have just had a quick look and there are three available for this year.
Arcadia (Child Free) departs 26 Sept for 23 nights, Aurora departs 18 Oct for 22 nights, and again on 20 Dec for 22 nights. Cabins still available on all three cruises.
If you are thinking of 2009, again check on the P&O website, there are quite a few for next year, the choice is yours.
P.S. when you do decide to go, get a Cruise Agent to book for you they give better discounts than booking direct with P&O.
Best of luck and enjoy.