All adults are assumed in law to have capacity to make their own decisions unless it is established that they lack that capacity. Even then, people must be given all the help they need to make whatever decisions they can for themselves. Just because someone makes a decision another person thinks is unwise, that doesn’t mean they lack mental capacity.
The laws themselves are slightly different in England and Wales, in Northern Ireland and in Scotland, but they all have similar principles. All have similar tests for capacity, involving the ability to make, communicate, understand and remember decisions, as well as how this might be impaired at some times and for some purposes.
Anybody taking decisions on behalf of a person who lacks capacity must always do so in the person’s best interests.