I was in a similar situation a few years ago - just getting out of it now. I think you are going at this arse about face trying to spend money to save money. That’s a luxury you don’t have. Windows and solar panels and chickens take years to repay what you spend. You don’t have time to be arsing about with chickens. This is what I did
No unnecessary journeys that aren’t by foot. If your car is worth more than £10k and gets less than 60mpg then get rid of it and get a decent ish cheaper more economical car. If your car isn’t worth much then it’s better the devil you know.
No “nice to have” subscriptions. A tv one, fair enough if you watch tv but you don’t need lots of them. Watch Netflix to death and then cancel it and get prime or Disney or whatever. Not sky - it’s too much. I didn’t have anything but bbc for years - ITVX and channels 4 and 5 have loads of free stuff. I paused my audible and used borrowbox and re listened to stuff I’d already bought. No gym or sending you face cream or magazines through the post.
No expensive hobbies. Brownies is ok, horse riding isn’t. Adult hobbies can be things like yoga in the house, running about the streets in trainers you already own, reading library books. Under no circumstances allow your husband to take up triathlons or golf.
A 21 day meal plan repeated until the end of time - made shopping and cooking easy. You have to be smart about it. You want oily fish every week? You can’t casually grab £10 worth of salmon anymore so you do mackerel with leeks and baked potatoes, or tinned sardines in tomato sauce with pasta. You need fruit? Buy whatever is in the super 6 in Aldi or whatever is in season or inexplicably cheap. Apples and pineapples and bananas and oranges - not raspberries etc. You do chicken wings, not steak, pork shoulder, not chicken breast, lentils and chickpeas instead of lamb. Cheap biscuits instead of home baking. No juice or pop - but I never stopped buying coffee. Remember mince and dumplings, sausage and beans, egg and chips are perfectly good meals. Simplify your meals and try to get a decent amount of nutrition in over a week - not every meal needs whole grain, 5 vegetables and some seeds. I’m not against gadgets (I have a slow cooker, and an air fryer) but it’s only worth it if you are actually using them. You can get an air fryer in B&M right now for £30 - how often would you use it instead if the oven you already have and how long would it take to make the money back? If your meal plan is potato wedges 5 nights out of 7 then maybe buy if you are mainly eating pasta or burritos then it’s not helping you. Slow cookers are often cheap (middle of Lidl etc often has them) and cheap to run but unless you are using them they are a waste of money. Cut down on your basics if you haven’t already. Lidl do excellent spaghetti and 5kg bags of rice.
Summer is easy - winter is hard. You need to stay warm. Depending on how bad it is and how old the dc are then consider sharing a room, or at least only use 2. Your dc will be warmer sharing a room, or sharing a bed, or even sharing a mattress inside a tent in their room. I would spend money on electric blankets and throws because they do make an enormous difference. You need layers - leggings, tights, base layers with thick socks and onesies on top and a hat (fleece beanies are good). You will need to sleep in your hats so don’t go for big woolly monstrosites. You need reasonable coats. If you have a large kitchen then move your sofa into it and spend all your time in there. Be careful with your hot water - time it right so everyone and the dishes can be washed but you aren’t spending too much. If you are cold go out for a walk instead of turning up the heating. Get thick blankets or wadding to line your curtains and get curtains over your external doors. Go to bed early.
Vinted is your friend with growing kids - buying and selling, but don’t buy for the sake of it.
The only things I would buy if I were you (and I was, pretty much) is electric blankets and throws, onesies and hats and curtain linings. I don’t have a tumble dryer or a heater airer and I managed to use cloth nappies without moulding up my house.
You need to meal plan ruthlessly - food is a big expense with 5 mouths to feed - and you need to shop according to plan. There is not much you can do about your mortgage and council tax and you can cut frivolities and luxuries dead - the savings are to be made on food and fuel so concentrate on that.