You are only eligible for the Graduate Route visa if you complete a course resulting in an undergraduate, postgraduate or PhD degree.
A postgraduate certificate / diploma granted as an exit award in your postgraduate course is not the same as the Postgraduate Certificate in Education (PGCE) or Postgraduate Diploma in Education (PGDE) – they are both awards of teaching courses.
There are only a few exemptions to this, which can be found on our Graduate Route page.