Hi Dave
Let us assume that you have standalone licences. This means that you got your FAA PPL by sitting the exams and doing the checkride, and not simply on the back of your UK licence.
In this case, when you take to the air, you have to decide which licence you are going to be relying on. If it is the FAA PPL, the you need to obey all of the restrictions on that. If it is the JAA/CAA PPL, then the same applies. As a result, if you want to take to the air in the UK, and fly on the FAA PPL, then you can fly above a covered layer, even without an IMC rating. Try and fly in a G-reg plane outside the UK, and the FAA PPL is invalid, and you will be forced to rely on your JAA licence with its limitations.
If your FAA licence is issued on the basis of the UK licence, this does not apply. This is because your FAA licence is endorsed that you must "comply with the limitations of your underlying licence". That would include not flying VFR "over the top".
Charlie
x