A diagnosis of dyslexia does not automatically mean that a pupil will have additional support, extra time in tests, a reader, a scribe, the use of a computer, or any of the other facilities that are available to those in greatest need. You may think it unfair when it applies to your child, but unfortunately, it is the case that year on year, budgets for SEND in particular, are squeezed until the pups squeak.
Whilst a diagnosis of dyslexia does not guarantee exam access arrangements, if their scores related to cognitive processing, speed of handwriting etc fall below a standard score of 85, they are entitled to extra time and/or other access arrangements in GCSEs and A levels.
If you are getting your child assessed from year 9 onward, work with school and get them to agree to use the assessor and complete the necessary part of the JCQ Form 8 before the assessment takes place. Most schools will do this provided they have had an opportunity to check my qualifications in advance of the assessment.
Dyslexia is a recognised disability under the Equalities Act and individuals in work are entitled to reasonable adjustments and are protected by law; university students with dyslexia are entitled to disabled students allowance and reasonable adjustments in examinations. It should not be any different for children taking exams at school.