Here's how it was in 1997/1998.
I seem to remember that you could drive on your original licence for a month if you're merely renting cars. They usually recognize the "International licence".
If you want to own a car you must insure it and to do this you require a Florida driving licence and proof of residence. By this I mean just a local adress, not residence permit.
The driving test is (was) a joke, you drive round a one-way carpark, looking out for all the hidden stop signs and old people, park up outside the office again and receive your licence five minutes later. Before the driving test you must complete two sides of A-4 questions about the rules of the road etc.. When I went there I wasn't even aware of the fact that there was a book to read so when I was asked if I had read the book my inner voice said "What book?" while what came out of my mouth was "Yes, of course". Don't worry, it's all so simple you'll laugh when you see it. If you've done a european driving test you won't have any problems answering the questions, it's all common sense. This doesn't stop some local residents failing five times but that's another matter.
Costs approx. 25 USD and you can go back as many times as you like. You only pay once you pass the driving test i.e. for licence issue. In fact they failed me first time because I didn't indicate enough in the one-way carpark. The next time I put the emergency indicators on full-time and drove very slowly. I even suggested that we could go out on the road to prove that I knew how to drive. The response to that was "Goodness no, have you seen how THEY drive out there?" Who would THEY be, the people who get their licences from you in this car park? I wondered.....
Insurance is dead easy, everybody goes for "No fault" insurance. Ask about that, not sure what this really means but it didn't cost too much for the old wreck that I had ($300).
Florida insurance is relatively expensive compared to other states because there are so many elderly people people who really shouldn't be driving but do anyway because it's the land of the free. More accidents that elsewhere.
If you've been insured for a few years (I think 4 years was the number mentioned) you can get a piece of paper from your insurance company in UK stating that you've been insured with them for so long and haven't had any accidents/made any claims.
This usually gets you a discount from insurers over there. Try it.
Of course the other useful side-effect of having a US licence is that you can use it to buy liqour and guns and ****. You won't be able to use your passport most of the time because it's not a "recognized" form of picture ID. Go figure.
Not a big deal, have fun.