Assistance dog or rescue - any advice?

Hi I haven't been on this site for a while but when I got my dog 4 years ago people on here were really helpful so I thought I'd ask another question and see if any of you have any advice or ideas.

My grandad suffers from dementia and is registered partially blind. At Christmas this year he came to stay with us and we found my dog really helped settle him if he was about to get upset. Every time he started getting agitated my dog, Sherlock would calmly go and sit next to him which worked as a distraction and stroking the dog seemed to calm him down.
We really think getting a dog for them would be a good idea as it would also give my grandma a companion too. She is finding everything very hard.
We are currently looking into the assistance dog options as there is a charity that trains dog specifically for people for dementia but they seem to very new and I'm not very hopeful that they will be able to help.
Our other idea was finding the right rescue dog that could be a companion for him but I don't know where to start and how I can make sure we get them the right dog. They are elderly so won't be able to cope with a dog that needs a lot of exercise though they do have a large garden and my grandma is happy to daily walks.
So my questions really is does anyone know of any other organisations or places that might be able to help or good rescue centres in the north of England that would be able to give us good advice and help us find the right dog?
Thanks for reading this