You are advised to register with the Disability Advisory Service, who are a central University support service who help advise the collegiate University on disability. They can create a Student Support Plan (SSP) for you (if you haven't already got one), and help you access reasonable adjustments, including exam adjustments.
If you have already got an SSP but it doesn't mention your need for exam adjustments, then you should get back in touch with DAS to discuss making changes to your SSP. You may need to provide additional evidence to DAS where a medical professional comments specifically on your need for the exam adjustment.
If you have provided the evidence for your need for exam adjustments and DAS are refusing to recommend them, you could consider submitting a complaint using the Universities Complaints Procedure.