OP, I honestly think that rather than saving £30 per month to spend on one day every year is not the best use of your money, if you don't have a huge amount of it.
I don't see why you and your husband have to spend £50 on your mum or his dad and stepmum. If you bought them a nice bottle of something each, you could get something for £12ish. M&S always have offers on booze and half-price gifts pre-Christmas.
£25 is a lot to spend on a niece (depending on her age, you could easily give her a £20 present but buy it for £10 if you look out for offers - Martin Lewis Moneysaving Expert has good ideas in the run-up to Christmas - or a £10 gift voucher if she's older. Or look on Facebook Marketplace/Ebay/charity shops).
£300 is a huge amount to spend on your daughter, even if you are sharing the cost (though how else would you do it, if you're married?) I know many, many parents who spend way more than that - but I also know many, many parents who spend way less. I aimed for about £100 per child (which I know is also a lot in comparison with many other people - but it's the amount I can reasonably comfortably afford - and also not an amount which would encourage the DC to think they can just ask for stupidly expensive items and be given them without having to do anything). I now give my adult DC £50 cash and a couple of little silly things to unwrap (always from charity shops).
I used to make cards, wrapping paper, tree decorations etc with my children when they were little as it kept them busy when it was dark at 3PM. We still use the decorations now, and the youngest is 17! They're still our favourite decorations.
Christmas dinner is just a normal chicken roast (it's just the four of us, so no point buying a big expensive turkey). I get it all from Lidl, and it probably costs a tenner with extras. Tree is a fake one which I bought about 20 years ago in John Lewis ex display sale for about £10.
Entertainment is free, as we play silly board games and go to Midnight Mass.
So it's not particularly expensive, but we have a nice time and the adult children still like it enough to come home!
My only real tips are:
Buy things as and when you see them, if they are bargains! 2nd and 3rd weeks of January are great for buying socks, gloves, hats etc with 75% off, and if the children are little, they won't care that they were 'last season' by the time they receive them. Ditto decorations, wrapping paper and cards. Waterstones have in previous years knocked 80% off cards in January.
M&S have fantastic 'reduced to clear' alcohol ranges. You need to keep a very good eye on them, but they make brilliant presents. I invested heavily in good vintage port which was reduced from £20something to £6. Their own 'posh' gin in was recently reduced from £20 to £10. Small whisky was reduced to £3, so that will be my dad's present. So keep your eyes open, buy stuff and store it!
It's a bit tedious, but it's actually no skin off my nose as I don't particularly set out to haunt the M&S wine aisles. It's just that if I happen to see it while I'm in there, I buy it and store it!