A very well placed and carefully thougt out question for a first post!! Enough there to make some people suspicious..anyway, in my humble opinion...
The main recommendation would be to delay your journey to the USA and ensure you have all the writtens done before coming out here.
"Groundschool" in the USA means "go home and read the book".
Air Law - slightly tedious and different in the USA from what you need to know in the UK (would be worth getting done and trying to keep at the back of your mind, as your USA instructor will probably have his blinkers on and not be aware that there are lands outside the USA).
RADIO written and practical - ENSURE you have both done before coming out. Go to your local school and sign up for a course of groundschool that terminates in passing the test. Some of the USA schools do not have R/T facilities and the tripe that they make you learn here will not work well for you when you return to the UK - the opposite cannot be said. The procedures you learn in the UK WILL work for 90% of flight operations out here.
Human Performance - rumoured to be relatively easy - crack it before leaving UK, just to reduce the amount required out here.
Aircraft Technical - later.
Meteorology - later.
Navigation - leave it till later if you have to - but navigation procedures are different here and I've found many an FAA instructor that does not know how to use the "wizz wheel" (so much planning being done on calculators and computers).
Which one have I missed - brain f**t going on here!!