As you have never had a SE Class rating you will have to go to an ATO or RF and do training to get a Course Completion Certificate so that you can take a SEP Class Rating Skill Test. As its an initial, the test has to be approved by the CAA, the school will know about that. I did one for a Cathay pilot about 18 months ago.
The CAA no longer provide guidance on this, its the ATOs job now, so don't waste your time calling them, you'd get better advise from a newspaper vendor on Paddington Station.