I would not see the point in doing this anywhere, other than the place in the UK where you are doing your training.
Lets say you have done 20 hours so far in the UK, you then decide to do some circuits in florida, you will need to remember the following things.
1. Return flights
4. Spending money
5. Car hire
6. Travel Insurance (not all bog standard travel insurance will cover you for flight training)
You will then on your return back to any UK flight school, will need to fly some more hours to refresh on your circuits to account for the airfield being used along with radio calls etc etc, so for your hours to count you would also need to use a JAA approved school.
If you have not started your training yet, it may well be an idea to go to florida and do the whole course in the USA, however like you say with regards to the exchange rates, it may well be cheaper to stay in teh UK.
Good luck in what you choose to do.