The first rule is that you shouldn't expect anything to be organised for you. There's no automatic routine, as each university has its own disability statement. Arrangements therefore vary from institution to institution. Your senior tutor can advise you on the normal procedure. You will certainly need to substantiate your claim with a medical certificate (in the case of illness) or an up-to-date educational psychologist's assessment (in the case of a learning difficulty). Do so as early as possible in the academic year, so that the exam board has time to make arrangements for you. (I recommend that you keep a photocopy of the certificate you send in case it gets lost in transit.)
Below are some typical situations for which special arrangements are normally made. To find out more about allowances that can be made for a range of conditions from dyslexia to blindness, contact Skill, the national bureau for students with disabilities, listed at the end of this book.
Examiners will be advised to make suitable allowances. For dyslexia-style conditions, this would normally mean disregarding spelling irregularities.