You can register the power of attorney as soon as the form is properly signed. Or you can wait until it is actually needed.
It’s best to register the power of attorney as soon as possible, however. That way, if there’s a mistake on the form, you can correct it before it’s too late to change.
It can also take some time for the Office of the Public Guardian to get back to you — better to register it early and have it to hand when you need it.