Not free but...
National trust membership is £10 a month and there’s loads of different places to take a picnic, look around a house and have a run around the gardens.
Treasure trails are about £8 and take a few hours. Good way to look around a local town/place but see it differently
Collect cardboard boxes and make a fort/castle/ship/den, kids can decorate it etc and as long as it doesn’t rain lasts for a few weeks of th holidays (or inside if you have the room)
Lots of mueseums are free
Geocaching
Look out for cheap deals, eg we did an escape room for £50 instead of £120 as we booked the 9am slot, often the early slots of things are cheaper.
Wacky warehouse do a summer pass , normally £20 for 3 kids and I think £10 for 1, you then have 6 weeks of unlimited use of the softplay.
Cheap morning cinema of kids films normally about £2 each and take your own snacks
Local wildlife places normally do activities during the summer. Things like pondskimming, big hunting, kite making etc
Sometime free or sometimes just a couple of pounds .
Local fetes, or country shows can be good to wonder and spend a pound or two quite cheaply.