Can you go to the US and do hours building with a UK obtained JAR PPL ?
No - you need an FAA license to fly an N-registered aircraft in the US. But you can get an FAA "license based on a foreign license" by doing nothing except for paperwork (and sending some money to the CAA). You will also need a BFR (Biannual Flight Review, I think) before you can use your FAA license, as you would with any other FAA license.
FFF
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