@Capricorn8990 sorry for the delay coming back to you. So take the following with a pinch of salt - this is just my experience and my Dad is years down the line from where your Dad is. We've seen the signs of memory issues for years, first classed as just short term memory loss but later as alzheimers. (Then 6 months later undiagnosed, then a year later re-diagnosed!! Did not help as in the intervening time my Mum did not get on any support pathway.) He did go on some meds but both of them didn't agree with him (made him depressed, other one gave him night terrors). Hopefully your Dad will get on better with his, they help to slow the progress. Does he remember his diagnosis now he's been told several times? My Dad was way worse when covid struck and he does remember that now - elbows only, no handshakes etc!
My Dad is still aware he has memory problems which is quite useful as we can remind him of that to help him put things into context. It took from 6 years ago seeing behavioural issues as well as memory problems (we had to manage him so he didn't get triggered by sensory overload from my kids boinging around) to 2/3 years ago he couldn't remember yesterday (even Christmas Day). Then 1-2 years ago my Mum and Dad would be driving away from lunch at our house and he would say 'you've got a lovely family' and she had to tell him but it's your family too. Whilst he remembered us in the house, he'd already forgotten who we were and who my Mum was as he was focussed on being in the car. Your Dad sounds a long way off this.
What I would say really helped was when my Mum, who was just honest to the core, learnt to lie to him. She struggled for a long time with lying, couldn't lie off the cuff and that caused friction. Much easier to go with the flow when you can with alzheimers, better for them, better for you.
Last summer my Mum very unexpectedly collapsed and died instantly and I had to drop everything and live with Dad for a few weeks until we had the funeral and got him a place in a care home (luckily we were pushed to the top of the list for a place as there was no way Dad could like on his own and there was a limit to how long I could stay there). It was such an eye-opener to live with him and see just how relentless he was, continually coming to find me when I was sorting things on the pc, asking questions, looking for Mum, not remembering she'd died "who died?" "my Mum died?" - no, your wife "my wife?" (can't remember wife) "but you're upset, who is she to you?" "need to put the outside light on Mum will be back soon" or, it would be like telling him for the first time and he'd be distraught. With the tunnel vision that his brain had, for a while he saw me as his carer, not daughter, as that was the role I was playing. That soon reverted once he was in a home. I do understand that the shock of the sudden bereavement had made him worse as well as suddenly so many things were different. My Mum would normally have been able to leave him for a few hours, go shopping see her friends, hide in my old bedroom and do craft things whilst my Dad would watch TV. There was some part of him that knew things were different when I was there.
So, when I say Mums don't tell you everything - that's what I mean. I often said to her that I could take him to his day centre and pick him up, or could babysit him - she didn't often take me up on it. I think to a degree she had got desensitised to it, also 'in sickness and in health' but I realised afterwards she must have been walking on eggshells a lot of the time to not trigger him and I'm really sad that I didn't know how bad it was to push her to accept more help.
Hopefully your siblings will step up in some way to support your Mum. I used to chat to my Mum daily so I knew lots of what went on, she didn't want to tell her friends as she thought they'd think "she's going on about her husband again". I don't think they would have. So I used to encourage her to chat about her day, what had gone on so that she wasn't holding it all in. Nothing either of us could do, apart from commiserate and sometimes we just had to laugh.
Anything to get the ball rolling with now will be a help to both of them in the future years. I can see that some people have given you other advice (I'm not in the UK so no idea about benefits etc) but search out what info you can as that will be really helpful going forwards.
Even though my Dad is now in a home I have started reading "Contented Dementia: A Revolutionary New Way of Treating Dementia : 24-hour Wraparound Care for Lifelong Well-being" which is looking quite good. I had never thought to read up on this before but wish I had.
In all of this, remember - this is slow progressing disease. Aside from getting the POA done, you have time to get your head around what this means for all of you and get things in place. Your Dad is in a good place that he is now diagnosed and both he and your Mum can start on a pathway to help them both.
If you want to message about anything please do. And remind yourself regularly to take care of you, you absolutely need to be looking after you too at this time x