I have vistied Florida many time, mostly for disney lol. But I would make a pros and cons list about why you wanna move there and about what uni you are likely to get into and course. Finish you GCSE's in the Uk, as they are still an education certificate and can be used to help get into uni in usa. In personally option would be to move to the usa after gcse and a level, its puts less stress to catch up on the usa system of learning as it is very differnt to the uk, as well as if you dont like being in usa then can easly move to a good uni in the uk.
Furthermore, Since you are from the uk and would have gsce's before going to the uk it is likely that the uni's there will see you as an international student even if you did your SATs in the usa. Also you dont need good SATs to get into a good uni most of the time its a good personally statement and extra stuff you do on the side like a sport, if you did a sport and were really good in uk and went for a uni in florida and they had that sport it is likely you would get in due to florida unis liking people who do sport.
THINK REALLY CAREFULLY!! moving before finishing you GSCE's can ruin your chance of getting into any uni in the uk.