Regarding U.K. Law, I took this from the Gov.UK website.
”When you’ve made your lasting power of attorney (LPA), you need to register it with the Office of the Public Guardian (OPG).
It takes up to 20 weeks to register an LPA if there are no mistakes in the application.”
NB. There is a fee to pay.
This certainly adds more doubt as to its Legal Status over here. However, the medical profession normally respect an adult’s signed statement of their wishes.