Not quite correct, a PPL Instructor needed a minimum of a Class 2 medical. An applicant for a professional licence required a Class 1 but, a BCPL holder restricted to Aerial Work only needed a Class 2.
The major stumbling block with the Class 1 was colour blindness and some CPLs were issued with a Night restriction to permit flight instruction only, no night or commercial operation. There probably isn't any reason to do that now as it can be done with a PPL.