It is the driver’s responsibility to inform the DVLA. Failure to do so is a criminal offence that could result in a fine of up to £1000, may result in a prosecution if you are involved in an accident as a result, and could invalidate your insurance.
DVLA state that “You must let us know if you have ever had, or currently suffer from, any medical condition that may affect your ability to drive. If you are not sure whether to tell us about a medical condition, your doctor should be able to give you advice”. The DVLA website: https://www.gov.uk/health-conditions-and-driving contains more information, with a comprehensive A-Z list of medical conditions.
If you need to notify DVLA, contact them by phone: 0870 600 0301, by email: firstname.lastname@example.org, or by post: DVLA, Drivers Medical Unit, Swansea, SA99 1TU. Give your full name and address (If the address on your licence is incorrect tell them), date of birth, driving licence number and as much information about your medical condition as possible.
It may well be possible for you to return to driving after notifying DVLA and they will advise you. They may require you to complete a medical questionnaire which must be returned to them within the required time limits. The questionnaire will ask for details of your medical condition and will ask for your consent for release of medical information from your doctors. Alternatively these medical forms can be downloaded from the DVLA A-Z of medical conditions website.
If you decide to stop driving or are advised by your doctor to stop you will need to tell DVLA and are advised to send them your licence. Re-instatement of driving entitlement following a voluntary surrender of your driving licence is a much easier process than attempting to re-apply for a licence following a compulsory revocation by DVLA. Download the declaration of voluntary surrender form from the website, https://www.gov.uk/government/publications/declaration-of-voluntary-surrender, fill it in and send it to DVLA along with your driving licence (both parts).
The address for DVLA is on the form.