That £500 you've saved will buy you an extra 3.5 hours, near enough, at the £140/hr you quote for your home airfield. That might just about be enough to introduce you to flying IFR in Class G when you get home - I don't really know, because I haven't spoken to anyone who's done that transition. Also, I would recommend searching these forums for information about OFT before you commit to anything - lots has been written, and you may change your mind after reading it.
I presume, since you are posting in the Wannabes forum, that you are a Wannabe? In which case, my advice would be to do the IMC rating at home. Then, go abroad to hour-build, and to do the CPL. There is a good choice of CPL schools in America if you fancy going there. And for your hour-building, you can go anywhere in the world that takes your fancy.
There is some great flying in America, you are right to want to try it - but not right, IMHO, to choose the IMC rating as the reason for going across the pond.
FFF
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PS - I thought you need to be 17 to do the skills test for the PPL? Or can you do the test before that age, but just not apply for the license until after your birthday? Just curious.