Try vacationstogo.com

You can sign up for their offers so you get cheap deals as soon as they appear.

Hope that helps and good luck.

Book direct from Royal Caribbean- or take a look at one of the online US booking agents- Orbitz/ Travelocity/ Expedia/Hotwire.... Royal Caribbean has the Sovereign of the Seas sailing from Port Canaveral in Sept 08- prices for Sept 5th/12th/19/26th start at $219 for inside and for the 4 days it starts at $249. I have cruised on this ship and it is excellent. I don't think you can beat Royal Caribbean for service and fun and with these prices!!
The only problem is you won't want to get off after three days. This ship alternates 3 and 4 day trips. On the 3 day she goes to Nassau, Cococay (private island in the Bahamas) and on the 4 day the extra day is spent at sea.
If you like Carnival they have the Sensation that goes to the Bahamas from Port Canaveral,but that ship doesn't compare to the Sovereign.
Hope this helps.

BTW- for the price Thos Cook has quoted you, you could get a suite on Sovereign of the SEas in Sept.

Try Walmart (travel ) , Allseasons, Costco ...and for Disney cruises that also go to the Bahamas from there ( and which are highly recommended, but pricier than the average cruise) Dreamsunlimited.

These are all established US firms - and hence you can now get great prices with the current $/£ exchange rates. With most US firms they also have very generous full deposit back terms up to about 3 months before sail date, and for many cruise lines, the earlier you book, the better the price and more choice of cabin location.