Echo BillieBob - there is no "aerobatic rating" in UK so there is no need to be qualified as an "aerobatic instructor" to teach aeros. An instructor and student can do whatever they want in a suitable aircraft as long as the student is willing to pay.
The "aerobatic instructor" restriction is only really relevant to countries (such as France) which have an "aerobatic rating". To obtain an "aerobatic rating", a student has to be taught by a suitably qualified instructor.
But it is still all a bit daft really. Even in France a PPL can be taught aeros by an "unqualified" instructor, and then go and fly aeros solo or even with pax - who is ever going to know? There are plenty of ex-FAF pilots topping up their wallets at aero-clubs over France instructing aeros, many without even a basic FI(A) instructor rating.