Book an assessment on your boat or if you have a PB2 certificate fill in the form that you can download from
RYA - The Royal Yachting Association | RYA send it off with required fee there you go , you don't have to join the RYA but it works out cheaper if you do and valid for 5 years.
If you go the BSAC route you will still need to join and then join a club and then try and get on a course.
IYT not huge in the UK and would not know where any test centres are.
Tim