And that, in turn, will require that you've completed something along the lines of the AOPA Aerobatics syllabus, or the lesson plan that's in Robson, or something similar.
I haven't done either. My understanding is that to claim grandfather rights, you will need documentary evidence that you have done aerobatics. Nothing about what training you may have done. Hence entry into a documented aerobatics event will suffice.
BTW, you won't need this rating if you fly an LAA A/C on a non expiring UK PPL.
This very much based on other countries who currently have aerobatic ratings and who are insisting that they exist under EASA. There is no evidence to show that this will add anything to safety and will just add cost as I'm sure they will charge money to get a new piece of paper printed out.