My Dad had Vascular Dementia so I got the postal vote for him.
He still watched the news and took an interest in politics, even though he had a 30-second memory.
I used to wait for one of his 'better' days and ask him who he'd vote for these days, as he'd always been a Labour man. Once he'd seen the candidates on the news, his memory was refreshed and I could ask him, "Would you vote for him/her?". I was then able to fill in the form and luckily, he could still write, even if the signature was very wobbly.
I can understand that a lot of people are in a worse situation. I'd just leave and not bother to vote unless I knew for certain what their wishes would be.
My sister mentioned David Cameron at the time, but he couldn't remember who he was. She said, "He looks like a giant pasty." My Dad replied, "Oh, yes, I know who you mean. No, I wouldn't vote for that idiot!" Still makes me laugh!