I have just checked my FARs and this is what they state
FAR 61.57(d)(2)
The instrument profiecency check must be given by
1) an examiner
2) A person authourised by the US armed forces to conduct instrument flight tests, provided the person being tested is a member of the US armed forces.
3) A company check pilot who is authourised to conduct flight tests under Part 121, 125 or 135 of this chapter or subpart K of Part 91 of this chapter and provided that both the check pilot and poilot being tested are employees of that operator or fractional ownership program manager as appliacble
4) An authourised instructor
5) A person approved by the Adminstrator to conduct instrument practical tests.
No mention of:
somewhere in the FAR that it may be renewed as long your foreign (Non FAA) instructor or examiner is recognized by your authority, in this case the UK CAA; the flight test certificate must be recognized by the FAA.
I would email the FAA and get it in writing as currently I don't think this is the case, you will need an FAA bod to undertake it for you.
J.