These people might be able to help
There are also a fair few CFIIs around the UK working freelance who may be happy to help, and may charge a fair bit less than £60+VAT.
A quick search of the FAA website brings up the following examiner:
HUGHSTON, THOMAS P
39 SYDNEY DYE COURT SPORLE
KINGS LYNN NORFOLK *PE322EE
UK
Phone: 011 447710564421
FAA Office: EA15
Aircraft: CE-421-421, DA-42-42, PA-34-200, PA-44-180
Also, see here for a link to §61.65
Instrument rating requirements to see exactly what boxes need ticking.
All the info on FAA practical test standards, rules and regulations is all freely accessible on the Internet. Also worth checking what TSA hoops need to be jumped through, and how to go about getting DfT permission to use your N-reg for flight training (where an instructor is paid) here in the UK.
Personally, I'd just go and spend a month in the States with a reputable outfit such as
Chandler Air Service and tie it all up over there.