The best thing to do is to register as a private candidate. You'll need to find a centre(school or college) to provide you with a desk and seat to sit the assessment come exam time. Both the examination board and centre will charge you a fee.
It is up to you to print of the specification and obtain past papers.
The best thing to do is to contact the LEA(local education authority) and ask them for a list of centres within a closeish proximity to your location.