7700 - as a JAR PPL holder you just need to do training at the judgement of the FI/CRI to pass the TMG class rating test. A few hours should be fine. Pass the test and pay the rating addition fee to the CAA and you then add a TMG rating to your licence. A TMG rating is the appropiate JAR-FCL rating, it covers SLMG privileges. You need 15 hours on class to exercise instructional privileges in that class (see LASORS Sect H).
If you can wait to post-end Aug I should be able to do your test if you need someone.