Can I still advertise that my product does not contain a banned substance?

No, where the substance is already prohibited by law for that product category. Presenting legal requirements as a distinctive feature of your own offer is always impermissible.

The claim remains permissible if it goes beyond the legal requirement, for example through a wider test scope or a stricter limit, and that difference is stated.

The difference has to be named and demonstrable, not merely asserted. A broader scope of testing or a stricter limit than the statutory one carries the claim where it follows from the certification programme and the result has been verified on the product itself.