It really does depend on the individual and where they trained ... you don't say but since you're mentioning a 3 week JAR PPL, I'm assuming that you went to one of a handful of large Florida based schools, the ones that run the biggest adverts in the back of Flyer every month?
If that's the case, then depending on the individual you could get away with a general handling flight and a couple of quick dual nav's, or virtually end up redoing your entire PPL. I have flown with people that basically had the skills and just needed a bit of practice with British R/T and our somewhat more sporty weather, and on one memorable occasion I also had one Florida PPL that I could swear had never been inside an aircraft before in his life if he hadn't had the licence in his hands. That was a case of another 8 or 9 hours just in the circuit trying to undo the damage that had been done to him, before I'd even let him go solo, let alone try and sit an LPC ...
I'm not implying that you particularly suffer from any of these things, but in general some other common symptoms of '3 week Florida PPL syndrome' include sloppy and lightning fast checklists, poor lookout, rough balance, taxiing everywhere at a million miles an hour and the legendary American R/T
Things you might be able to do to save a bit of time and money - get hold of the UK CAA R/T manual and have a read. If you are mates with any British trained PPLs then get them to sit down with you at the coffee table and be an imaginary controller while you do an imaginary nav ex with them. If you're not totally comfortable with the checklists and if your local flying club will let you, then go sit in a parked aircraft and make sure you can do the checks verbally, 'exactly as they're written in the checklist', without cutting any corners. Get hot on the principles of basic stopwatch-and-heading navigation, and don't freak out the first time you have to go on a nav to a place that doesn't actually have CAVOK, a 2 mile long concrete runway, and six different approach aids on field
That's probably more advice than you wanted to hear, good luck with it anyway mate.