But is it legal? Unless holding at least a CRI Rating, there is no legal way for such instruction to be given unless someone very brave at the CAA has issued some form of exemption.
One way to get around this legal hurdle is for the 'student' to log all time as the PIC, while the check pilot is officially a mere passenger. This is not a perfect solution, but workable.
To my mind, anyone giving aerobatic instruction to a licence holding pilot must be AT LEAST a CRI(A) - and if charging for such, must of course hold a Professional Licence.
Well, you are entitled to your opinion. Personally, I'm inclined to think that the only real criteria are experience and competence. I know several high-time pilots, mostly ex-military, who have no civilian instructor qualifications but are more than capable of serving as checkpilots. And I know some civilian instructors who have lots of paper qualifications but not the matching skills.
IIRC,
Duane Cole lost his medical about ten years before his death, but continued to give informal aerobatic instruction ... and his services were sought-after by those in a position to know.