I'm not sure if you can do it in the UK. There is a school (see one of the other posts about FAA schools in the UK) in Norwich which can carry out the FAA lazergrade exams, but I'm not sure they can issue the airmans certificate. You may be able to get the exams done, then take your results to a FSDO in the US when you get your FAA licence, and then they'll issue the certificate with IR privileges straight away. Try giving them a call and see whath the score is.
Cheers
EA