Home educated children exams as private or external candidates. It is up to the parents to find an exam centre. A few local authorities MAY be able to suggest a suitable centre. My main page on exams is here
Access Arrangements are pre-examination adjustments for candidates based on evidence of need and normal way of working and fall into two distinct categories: some arrangements are delegated to centres, others require external approval. The rules are set by JCQ. JCQ Access Arrangements Private Candidates via this page
The normal way of working within the centre is a critical issue for home educated candidates because there is no previous relationship with the centre as there would be for a pupil in school.
There is no obligation for centres to take private candidates, let alone those who need extra help, ie there are no grounds for a disability discrimination case if a centre says it can’t help. There is also the issue of who carries out the assessment. Sometimes people think that they just need to take a report to the exam centre so for example they pay hundreds of pounds for a dyslexia assessment, but that’s not how it works.
Ideally you should be talking to exam centres the year before the exam. If you know that your son or daughter will need the standard set-up to be adapted, for example because it takes them longer to process instructions or they would get too tired writing for all that time, or not be able to see the paper properly to read the questions, then you should be talking to exam centres at least a year before, to confirm a/ what type of specialist report is required and b/ to negotiate how the centre will establish the “normal way of working within the centre.” Some parents choose the exam based on which exam boards are preferred at a supportive centre. More about exam centres and much more on the home ed exams wiki.
Access Arrangements can include extra time in exams; rest breaks during the exam; permission to use a word processor in exams; someone to read out the questions in the exam; someone in the exam who will write out the answer dictated by the candidate. The RNIB has a useful page on access to exams and tests for the visually impaired here.