To do the ground exams you have to go through a CAA approved RTF that has a Ground Examiner. I am also GI/GE and official keeper of the CAA exam papers.
So you might be able to do the test with a free lance Examiner but you will need to find an RTF to sit the papers.
The easiest way is to get 100hrs sit the RT and Air Law exams and do the test, saves having to learn the whizz wheel for a start!