Power of attorney
A power of attorney is a legal document that allows someone to make decisions for you, or act on your behalf, if you're no longer able to or if you no longer want to make your own decisions.
An ordinary power of attorney is only valid while you have the mental capacity to make your own decisions. If you want someone to be able to act on your behalf if there comes a time when you don’t have the mental capacity to make your own decisions you should consider setting up a lasting power of attorney.
https://www.ageuk.org.uk/information-advice/money-legal/legal-issues/power-of-attorney/