Mate as far as I am aware and I stand to be corrected but I dont think you can do it that way .
You have to be on an apporved course of either full time or distance learning study . That is because both includes elements of progress tests and mock exams of which you have to complete prior to being allowed to take the exams . The DL phase also requires a specific time in the classroom prior to taking the exams .
I am not aware of anyone taking these exams by just signing up and turning up .