There are loads of different sorts from the 'flash' ones (Princess, Royal Carribean, P&O etc) to the cheaper less formal ones (Island Escape etc). When booking, remember an inside cabin will be miles cheaper than an outside one with a window. Lets face it, hopefully all you will do in the cabin is sleep!! If you have a search on google there are loads of firms doing cruises. Personally, I quite often go with Airmiles. If you have some Tesco discount vouchers, these can be converted to �4 for �1 vouchers to use with Airmiles (It all adds up!) and Airmiles can usually do you a good deal. If you want to while away some time going through reviews etc, look here.
http://www.cruisecritic.com/
Another bit of advice. It is a long way to go for a week, so look into ships that do 'back to back' cruises. (2 1week itineries from the same home port) This works out well as a lot of the cost of your holiday is taken up by the air fare (which will go up if the Government have their way!) Go if you can...I love cruises! (2 booked for next year...can't wait!)