You can definitely go to the UK and do the exams there. Since OFT uses the same CAA exam papers, you can simply take the paperwork that shows your "pass" to OFT and they have to accept it.
However, at OFT the first exam attempts are included in the package deal and I can imagine they're not crediting you for the exams you've done already. But if you wanted to do them beforehand you've got to travel to the UK, maybe get a hotel, do the exams at a flight school and pay for them, and then travel back. I would prefer to spend my money and time on something else.
In any case, make sure you're 100% done with studying before you travel to Florida, and try to pass all exams in the first week. If you consistently pass the confuser/perfector/simplifier sample exams with about 80% or more, you should have no problem with the real exams.