TSA clearance
187nj,
Not sure if anyone has mentioned this, but make sure you get complete TSA clearance before you go to America. I had most of it before I went, with only the fingerprints to be done when I arrived at the flight school. I think generally it takes around 3 days to get the Fingerprints approved, but for me it took 10 days. Obviously if this happens to you then you'll come home with a lot less flying than planned.
I'm pretty sure you can get the fingerprints done in the UK before you go, (not exactly sure where but farnborough rings a bell, perhaps someone else can comment?) At least then you could start flying pretty much as soon as you arrive.
Also, if you havent booked your flights yet, I would say to try and give yourself as many extra days as you possibly can, 21 days to complete a PPL really is the best case scenario...
Good luck
Stu