"When it comes to writing death certificates in the UK,
there is strict government guidance that says doctors should avoid giving old age — or even 'senility' or 'frailty of old age' — as the sole cause of death. There are, however, "very limited circumstances" where this is acceptable. These are when a doctor has:
• personally cared for the deceased over a long period (years or many months);
• observed a gradual decline in the patient's general health and functioning;
•
not been aware of any identifiable disease or injury that contributed to the death;
• been certain that there is no reason that the death should be reported to the coroner."
https://www.walesonline.co.uk/news/health/can-you-die-old-age-25139181
I am not certain why Mr. Brandreth would want to get the Queen's physician in trouble, or cause yet another source of denial for the Palace.