That is a very difficult situation.
When families are close, it is just a no-brainer that the offspring ( or rather some (rather than all) of the offspring) take on the not insignificant burden of dealing with their parents/family. There is an expression that each person chooses to take on the level of responsibility that they feel comfortable with - for some people that is everything and for others its enough to make the occasional, casual enquiry.
It is, of course, possible to simply refuse to have anything, at all, to do with your father. I'm not sure that anyone can, legally, compel you to take any interest at all. It is a fairly ghastly reminder of how badly we all need support from family and/or friends when it all hits the fan. I suspect , were you to do that, that your father would be moved to a hospital bed with responsiblity, ultimately, ending with the HSE.
If you are accepting responsibility then, unless the UK pension people change their minds and will deal with you, your only course of action now is to have him made a Ward of Court with you as the appointed Committee i.e. you have authority to deal with matters on his behalf under the watchful eye of the High Court.
As your father is not compos mentis, it is not possible for him to appoint you his Attorney.
mf