2D,
Thanks, that helps a lot. The key point for me in your reply is that it is possible to add an IR to the "based-on" licence. I'd been told that wasn't the case. I already have an FAA Class 1 medical.
I think it may boil down to a question of timing. The UK based school can't commit to when an appropriate examiner can visit the UK for either my PPL or IR tests. I'd like to get the whole thing finished by the summer so I'm beginning to think that the best way is to head over to the US and to the training there.
H