Bubbles, flights to the Carribean are quite cheap, you can find self catering accomodation for most destinations advertised on line as well, why not do the leg work yourself? You wont have a rep or travel agency to go back to if it goes wrong or if it isn't up to expectations, however I reckon you will save money, and will also have the freedom to eat where you want, choose which dive operator you go with etc. It's worth grabbing a lonely planet guide for the destination you want to go to as well, the books may have been started for the backpackers market, however the accomodation listed generally covers a wide range of budgets, from the basic hostel to the 5 star hotel.

Also, get in touch with dive centres in your area to see if they have anything booked, or would be interested in setting a group trip up for you, take advantage of the discounts that will bring. There's quite a few in Reading, also Aquatic Element (www.aquatic-element.co.uk) has recently opened up in Windsor.