It is my understanding that the person for whom the LPA has been registered can conduct their own affairs with no regard whatsoever to the person who has the LPA.
I do not know, but suspect that it would have to be 'proved' by the LPA holder that the person was 'without capacity' before any decision could be taken overruled.
I believe that a 'suitably qualified' doctor would have to submit a report to the Office of the Public Guardian. In most cases that would be either a psychiatrist, a geriatrician , a geriatrician who specialises in psychiatry, or a psychiatrist who specialises in Old Age Psychiatry.