Often when people are looking to buy property in Florida they believe it's difficult to move to the U.S. legally or to be able to spend more than six months out of the year there. However, in reality the U.S. has some of the most liberal immigration policies in the world – it’s just a matter of knowing how they apply and what you have to do to qualify.
While the U.S. does not offer a "retiree" visa per se, there are a number of other options available specifically to UK Citizens looking to buy property abroad.
Family based residency
If you already have immediate family members who are residents or citizens you are likely to be entitled to residency in the US. Immediate family includes parents, children and spouses. Depending upon the status of your sponsor (resident or citizen) and the relationship with them it can take more or less time to obtain your residency. There is no waiting period for someone marrying a US citizen or for the minor child of a US citizen, though there will be a processing period that can be 12 months or more. At the other extreme, brothers and sisters of US citizens may apply but there is a 12-year waiting period!
The B Visa (Tourism/Business)
Most UK Citizens travel to the US without a Visa. However, if you go through the trouble of obtaining a "B" Visa, you can stay for up to six months at a time. If you don't abuse your visa privileges, you can actually spend more than eight months per year, provided you break up your visits in smaller fragments of say, four months each. What you will not be able to do is spend six months, go out for one week, and come back for another six months. If you try doing this you run the risk of having your visa cancelled.
There are also student visas, visas for those going to work in the US and even an annual lottery although this is rarely the best chance for obtaining residency.