Health Assessment

At my health assessment, I was told that my prescription’s validity for the drug test MAY not be viable as i’m not prescribed it on the NHS. I was also told that they don’t even know if my prescription letter is from a real doctor because anyone could put “signed by dr -” if they have a doctorate of any sort.

I’ve been told to contact my GP to get a letter from them to confirm my medication to help with my evidence.

Does anyone have any resources to help me with this? Is my prescription less valid because it isn’t NHS (in terms of a safety critical role). Can I be told my drug test can’t be backed up by a private company’s prescription?

I will get a letter from my GP stating that they approved my cannabis but I was wondering if anyone had ideas for more things I can bring to my appointment.

Or even helpful links for this situation,

Thank you!