I am not so familiar with using an EPA though my mother in law had one. An LPA can't be used at all until it is registered with the Office of the Public Guardian, but can be registered while the person concerned (the donor) still has mental capacity, at which point it is up to the donor to choose what they wish the attorney(s) to do and when.
Since it can't be used until registered, some people advise making an LPA well in advance of when it might be needed, which can be done easily online. Once a person has lost capacity they cannot make an LPA or choose who will manage their affairs, it will be up to the Court of Protection to decide who can act on their behalf (a Deputy).