Bahamas makes sense, you have to think of us UK guys too, mexico can be a pain especially cabo san lucas without going thru the states. London flies straight to Nassau so that works.
As for hotel choice, I went to Atlantis for an affiliate event a few years ago and it really is an experience. Yes some of the rooms are pricey but in this case you really get what you pay for. Everyone is on paradise island which makes sense, the casino is fine, and theres a couple of bars / nightclubs to enjoy, not to mention the restaurants.
Most casino affiliates have the cash to treat themselves once a year, and those that dont could always speak to an operator 