Where a person has capacity for a decision about specific medical treatment now, they can make an Advance Decision to Refuse Treatment in future, to be applied if and when they have lost capacity. There may be formal requirements depending on the kind of treatment, and it is prudent to get legal advice.
Alternatively, the MCA allows for a person to appoint someone to make decisions on their behalf in future (a Lasting Power of Attorney, or LPA), and this can apply to decisions about property and finances, or health and welfare, or both. Again, there are some formal requirements, and it is a good idea to take legal advice. This is for someone who has capacity now to appoint an LPA, to identify a person who they want to be the best interests decision maker after they lose capacity. But that Attorney will still be bound by the MCA – ie they can only make a decision in the person’s best interests, and under the same duties of consultation with others.
In many cases, of course, a person may have already lost, or never had, capacity to make an LPA or an Advance Decision to Refuse Treatment, and so these options are not possible. In that case, the Court of Protection has power to appoint a “Deputy” as a best interests decision maker. Again, any Deputy is limited by the requirements of the MCA and it is relatively unusual for the Court to appoint a Deputy for health and welfare decisions, including serious medical treatment, as it expects these in any event to be taken collaboratively by the clinicians and the family and others engaged in caring for the patient or interested in their welfare, with any serious dispute to be referred to Court.