Taking all or some of your exams before you go to the States, will certainly give you an easier time & a chance to have some holiday also.
However, the point of this thread is the cheapest way to gain a PPL and you are unlikey to get much if any, of a discount from the course price for having done the exams. Certainly not enough to make up for the money you've spent on tuition, books, study aids & exam fees in the UK. All of which are included in most US courses.
Doing everything in three weeks means long hours and hard work. But, I had a great time.