I think its getting clearer now, but not totally. FAA tell me that I must have an FAA PPL. I don't believe that this can be a validation PPL now though, because the FAA are restricting adding any other rating to a validation. This would mean me sitting the written exams and having three hours with an FAA instructor and sit the FAA skills test to get my PPL.
I THINK that my prospective school had an agreement with thier 'roving' examiiner. That being, if the PPL writtens had been completed, he took your CAA PPL and 250+ hours as good as a FAA PPL skills pass.
But I stand ready to be corrected.
LF