Don't have pets - bloomin fortune.
Charity shops for clothes and household items (I've bought tops/bottoms for £1, eBay is going to be at least £3.20 for postage so charity shops is the way to go, unless it's a posh charity shop that charges too much, I tend to avoid my local British Heart Foundation as found this was the case).
Save bath water and use a jug of bath water to pour into the toilet instead of flushing.
Instead of donating old/outgrown clothes, cut them into pieces to use as toilet paper. I'm not joking, toilet paper is expensive.
Replace tampons with a re-useable menstrual cup.
See if there are any benefits you could claim.
I bought a cargo E-bike to save on bus fares, car fees, parking fees and to cut down on the time is takes to walk or bus everywhere (I'm 30 and never owned a car even with two kids). Also enables me to do a big shop in one go without getting a taxi.
I sold my washing machine and tumble dryer and now use the free on-site laundrette attached to this flat.
Switched from liquid to the cheapest laundry powder.
Don't use the storage heaters, bought other heaters and only switch them on when I am actually freezing or to warm kids rooms up before bed.
Haven't had a holiday in over 10 years.
Bring a flask of coffee with you if friends want to meet for coffee. This works for me as I have a pram to store it in.
If your baby had eczema try to figure out the cause rather than just treating the effect, as it took a lot of back and forth to the doctor but figured out he has a dairy allergy that causes him to flare, doctor prescribed dairy allergy formula so thankfully don't have to pay for it any more. Of course breastfeed if you can, I can't.
Cut down nursery hours for children and only have them go for the free 15 hours a week. I've saved £255 a month doing that.
Don't be a smoker of any kind.
I have a quick shower once every 3 days if my hair can last that long. Try dry shampoo. Kids have baths together.
Disposable nappies during the day. Again, only worth doing because I have free on-site laundry.
Yellow labels and freeze what can be frozen.