cat - does your Dad attend hospital or clinic etc - not sure what his illness is, but if your GP knows you have been caring solely for your Dad with no outside help, I am surprised they have not suggested something before now.
Social Services will do an assessment of your needs as his carer - by the way does your Dad receive carer's allowance? if not, he is entitled to this.
I think it also depends on what area you live in - some boroughs are better than others.
My Mum is 81 and has vascular dementia - my Dad who is 80 is her main carer, with me going over to them 3 times a week. I know how exhausted you and your partner must be. After a long and drawn out process, social services have allowed my Dad a carer to sit with my Mum while he pops out to get some shopping but only for 3 hours.
You are so young my love to take on caring full time - I take my hat off to you. I know all about the feeling guilty, I feel like that every day and feel I am not doing enough.
I am an only child, so there is no one else to share with.
I may be going down the respite care route very soon with my Mum, so I know exactly how you are feeling.
There is help available cat, its just a question of putting the wheels in motion.
Good luck - you are one lovely caring daughter.
Den xx