If you are doing an FAA PPL then you need to sit the FAA written exam - just the one exam, I think, unlike seven for the JAR-FCL PPL(A). You could do it in the UK, but as it is a single exam it's probably easiest to prepare before you go and sit it once you're there.
Plenty of study material ->
http://www.sportys.com/students/
You'll also need to get a medical at some point; worth looking up the requirements and making sure you meet them before you go.