It's a very good idea to get the PPL theory out of the way before going to USA. Just need to get the books/ confuser / past papers and study in rainy old UK, then you will have lots more time to get practical in the sun
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You will have to find a local FTO that is willing to let you sit the theory exams. Many might be reluctant / indifferent as it is not a terribly profitable activity for them.
. If you are going Commercial maybe able to persuade with promise of future training.
good Luck
Fran