I "work it out" by having done it successfully. On Page 5 of the instructions it says you must indicate on page 2 of the form why you are signing for someone else. Among reasons you can sign are because you have an EPA, but also if the person is too sick to sign for themselves and several other reasons. I used the "too ill or disabled" option with no difficulty. In the OP's position, I think I'd call the DWP and actually ask. I have found them quite helpful (very helpful compared to some bodies) making the point that you are an appointee and that you want to spare your father distress by not push his deficits down his throat.