I'm sure that it's indeed possible to do this.
But if anyone elected to take the exams at my RF without intending to do any flying training as well, they'd be required to pay the one-off joining fee and would be charged the same as the CAA charges for sitting the exams.....
Whereas if they do their flying training with us, the first attempt at any exam is totally free of charge.
Use a UK RF/FTO for your exams, then bug.ger off to the US to do your flying? Yes - but expect to pay!