But as pointed out earlier in this thread, an instructor rating is only required to give instruction for a licence or rating, and as there is no such rating you can legally do it on the basis of your licence alone, if its a CPL you can charge.
Trainingcom 1/2005 states:
AEROBATICS ON TRAINING FLIGHTS We would like to remind you that the inclusion of any aerobatic manoeuvres during training flights constitutes aerobatic instruction, whether these manoeuvres are part of the training syllabus and the intended lesson or not. An Instructor is exercising the privileges of his/her FI rating throughout a dual flight (including a trial lesson) and to carry out or demonstrate any aerobatic manoeuvre, therefore requires that the Instructor’s FI rating is not restricted for the purpose of giving aerobatic instruction
Whilst this statement is not entirely true when instructing a qualified pilot, and it doesn't change the legality, but it does make sense.