Hi Looper,
I would certainly recommend you study for and pass the written exams over here in the UK before travelling to NAC for the flight training course as it takes the pressure off somewhat.
One word of warning: the exams you sit must be the UK CAA exams and not any other JAR countries exams. The reason is because NAC is a JAR school approved by the UK CAA and thus they'll expect you to have passed the UK version of the exams.
Also ensure you take the PPL Application Form with you to the United States WITH the Exam results and endorsment from the Chief Flying Instructor / Examiner at the school you sit them at in the UK.
Whilst I was at NAC doing my CPL/IR there were a quite a few students doing their PPL's.
One student had passed all of his exams here in the UK before travelling out. He seemed very relaxed as he didn't have to do as much studying as most of the others who were doing their exams at the same time as the flight training course.
You'll have a great time over in NAC.
Can't wait until I go to NAC at the end of June for a FAA Multi rating and a bit of touring in their Arrow IV. I feel a lunch trip to Lakeland coming on.
Best wishes,
Charlie Zulu.