Highest qualification. Unless the operator is familier with all certifying agencies its a meaningless question.You never have to answer an inappropriate question, or a question you don't wish to answer.
A better thing to do, is just answer the question they should have asked. "What's your highest certification level?" "This is a 100ft dive? I've taken Advanced Open Water."
Now, if they're really insistent, which would be weird, "No, that's not what I asked, what's your highest certification?" To which you ask, "why do you need that information?"
Diving qualification, I'm a BSAC Advanced Diver - there is no PADI equivalent
Instructor qualification, I'm a BSAC Advanced Instructor – the nearest PADI equivalent is instructor trainer, but I’m not. It just means I can teach the BSAC Advanced Diver grade.
The situation I cites in post 53 came about because the front desk recorded my qualification as AI. On the boat the crew assumed I was a PADI Assistant Instructor. The fact I was wearing a polo shirt with BSAC on it didn’t register.