All brilliant advice here, Ruby. It is a worrying time but at least you are there to help your father and it sounds as though you will be there for him, no matter what!
My Mum has dementia and sounds like your Mum. She alienated my brothers by telling lies but, when she became ill we all rallied and I wouldn't cope without my brothers. I hope that can happen for you but, if not, remember GN is here with lots of experience and good advice. My brothers and I laugh at the things Mum says and does now, but it is not in a spiteful way. She is even quite nice to me sometimes.
If your father is going to the GP, could you get him to do a urine sample to take with you? The difference a UTI makes to people's memory is phenomenal.
I used to freeze portions of our food for Mum but, each time I spoke to her, she was eating cornflakes and I had to through vast amounts of frozen food away. If your father isn't eating well it is probably not a good idea to get involved in ordering frozen food for him yet. Meals on Wheels, as such, doesn't exist anymore (Health and Safety did for it, I think!) and round here accessing the new improved version is means tested I think.
Let us know how you get on and take care of yourself.