Protection of Personal and Property Rights Act 1988, s 94
For a personal care and welfare EPA, you’re “mentally incapable” if:
you’re unable to make and understand decisions about your care and welfare, or you’re unable to foresee the consequences of those decisions, or
you’re unable to communicate decisions that you make about your care and welfare.
For a property EPA, you’re “mentally incapable” if you’re not completely competent to manage your own money and property.