Your adult children who have moved out and have their own lives should be the ones hosting Christmas now and if you get along they should be inviting you and their brother to join them. If they could afford to move out they presumably have jobs even if it's only minimum wage. So should be able to afford to host a Christmas meal, even if it's not lavish and they also can't afford the heating. You're obviously too poor to do it any more. If they don't realise this, I'd be brutally honest with them about your circumstances. Let everyone know you can't afford to buy presents this year.
I wouldn't buy anything new but I would put up the decorations you've got, just to try to cheer yourself up a bit. Unless you really aren't bothered about Christmas decorations? I know some people only do it for their children. I like the decorations though and have always done a tree no matter how skint I am. I've still got the 4ft plastic one I bought 15yrs ago, the cheapest fairy lights and packet of baubles, a star on top. Some Christmases there's only been me here, I've still put it up because I like it and it makes me feel part of the festivities.
Your husband needs to stop being precious. As a household you need the money. If he wants to be a dickhead about not claiming something he's entitled to due to the effort, hassle or judgement then I think it's probably time to separate finances. The UC claim is a joint claim, so you each take 50% of it, afterwards putting the amount needed for rent and bills from your own account into a joint account that is only used for paying rent and bills. The remainder stays in your own separate bank accounts.
Your husband gets the carer's allowance into his account. For that he's supposed to be providing 35hrs of care a week so never feel guilty about asking him for help if you need it. If he doesn't want to do the job then he needs to not be claiming the carer's allowance and you'd need to get onto social services for an assessment.
You get your PIP paid into your own account.
You'd each buy your own food, toiletries, clothes, medication, phone package, gifts for others, travel costs etc from your own separate accounts.
If you separate finances you might find that although quality of life is poor, you've got some money left after bills for discretionary spending, whether that's on nicer food occasionally or something like being able to go swimming once a month or whatever.
Your youngest child I think needs an honest chat about the situation. Fact is you can't afford to sub them through uni. They have accommodation there, some kind of income or grant/loan they're living off? They needs to stay there in their accommodation and continue funding themselves. You can't afford to have them home eating your food, using the water etc and you can't afford to keep a bedroom for them when they've basically moved out already. It needs to become permanent. Let them know that they'll need to secure their own accommodation after leaving uni because you can't afford to have them back.
I know it sounds mean but it isn't really. It's ridiculous that we've become a country where going to university is seen as both necessary and something people are entitled to do. It isn't. Sometimes people can't afford to go and that's ok. Sometimes people make whatever sacrifices they need to make to fund their own way through university and that's ok too. Your child needs to face the fact you can't afford this so either they pay their own way as they are now or if they can't then they get a job to enable themselves to do so. If that means dropping out of uni then so be it.
If you're eating only two meals a week then you're basically starving and it will have negative impacts on your brain, you won't be able to think logically any more and will have a warped thought process. You need to eat more, end of.
You can create a situation where you can do that either by moving to a smaller property now all your children have moved out or renting the youngest one's old room to a lodger, ensuring you charge enough that it will cover the utilities they'll inevitably use. It'll mean more money coming in and being able to afford to put the heating on as well as eat properly.
Continuing on as you are and just cancelling Christmas as an added misery, isn't going to help you.