Ah OP, I've also been really down on my luck (and am heading that way again) and really sympathise. It's horrible when you can't give your children the things and experiences their friends are getting. Also, I see we're both in London and while we're lucky to have so much going on, it still costs money for a day out to even the free attractions. We used to go to the Kew lights as a Christmas treat but can't do it now.
What I will say, though, is that my mum was also a single parent who got no child support and raised us in an emergency accommodation council flat. There was never any money but I ADORED Christmas. I'm trying to think how my mum pulled it off and the most obvious thing was that our Christmases were rather quiet compared to most. A stocking full of silly little things, grilled grapefruit for breakfast (yes it was the 70s) progressing to bagels and lox when we got a bit of money, a walk, a fairly simple lunch, then presents from the tree. Then movies and games and snacks into the evening. Most of the day the TV wasn't on, and we didn't have many visitors.
I think the simplicity and quietness were key. It meant we didn't get overwhelmed and overloaded, and could really appreciate every nice thing the day offered. I still love Christmas, but really resist the overload: of prep work, themed activities, presents, people, food. Overload numbs us and leaves us jaded, needing ever-bigger dopamine jolts.
When mine were smaller, I used to build in a few little rituals that we would do together: a Christmas fair, Christingle at the local church, a visit to see Santa at the local garden centre where they also provided free seasonal craft activities. When we were at the fair I resisted pressure to buy tat by issuing a challenge: your mission is to find one thing, maximum cost £5 (or whatever), that I have to agree is good value. When we get home you have to wrap it and put it under the tree for yourself (or a family member). We also did lots of crafts (making Christmas cards, decorations, badly knitted scarves and peppermint creams).
So much of the Christmas magic is provided by fairy lights, hot chocolate, carols/Mariah Carey, and watching movies together. Do those things!