The "Golden Rule" sets out best practice when executing your will. In the case Re Simpson in 1977 Justice Templeman said the following:

"...that the making of a will by an aged or seriously ill testator ought to be witnessed or approved by a medical practitioner who has satisfied himself of the capacity and understanding of the testator, and records and preserves his examination or findings"

If you are very poorly, on strong medication, or if you have a dementia related diagnosis you should follow the golden rule.

The judge also included people who are "aged".  The term "aged" is problematic because it is hard to define. If you are in doubt it is best practice to err on the side of caution. Especially if someone might challenge your will.

At least one of your witnesses should be a medical professional who is qualified to make a testamentary capacity assessment. They must not only act as a witness - but they must make a report of your capacity at the time. 

Few GPs or family doctors are willing to do this. You may need to find a doctor or other qualified professional privately. This can lead to additional expense. 

Following the Golden Rule means that a challenge to your will saying you lacked capacity is more likely to fail. 

It is important to note that following the Golden Rule is not a guarantee of success at court. The Courts have final say on whether you had testamentary capacity. However, if a medical record exists it is extremely strong evidence that you had testamentary capacity. Any challenge to your will would have to present some extraordinary evidence to persuade the Courts that you lacked capacity.