What I described is what should happen, not what does happen. Because the UK CAA cannot be bothered to comply with the requirements of JAR-FCL, any FTO that holds an approval to conduct the FI(H) course can conduct an abridged course for the holder of an ICAO FI rating/certificate. This course need be no more than 15 hrs flight/30 hrs ground training, irrespective of the applicant's previous experience and does not need to be separately approved. Provided that you find the right examiner for the Skill Test, you will be guaranteed to get an FI(H) rating.
After next April, unless the UK takes advantage of the 12 months opt-out agreed at the last EASA Committee meeting, the CAA will have no choice but to comply exactly with the requirements and approve each course individually.