I can see what you mean - and I personally don't disagree - but the devil's advocate position would be that your job is to look after the children, and so you don't need the 30 hours free, IYSWIM?
A friend went through a battle with our LA about this, because they decided that she qualified for her biological child, but not for her fostered child, which was difficult because how do you explain that one child has to/gets to go to nursery for an extra day, and the other doesn't/can't?
There wasn't a way forward, though. She took the court case quite far but then ran out of funding. They told her that she was paid to look after the foster child, and therefore wasn't eligible for more funding to get someone else to.
I was fostered and absolutely see why you'd like the "time off", which isn't really off, as you've said - but I suppose if they were biologically your children, you'd have a lot of those same things, and wouldn't be eligible for help as you'd be a SAHP.
Did you talk to your LA about potentially continuing funding, if you think it'd be detrimental to drop it?
My son has just got his 30 hours free, although it now costs us more than it did when he got 15 hours free because they put the consumables and extra day charge up as they get less from the Government for 3 year olds 🙃 It's such a bizarre scheme. I know it's supposed to be optional charges, but you'd lose your place if you didn't pay here, and there's a massive queue for nursery places. Our wait list is currently until October '27.
Edit: i hope this doesn't sound dickish - I was trying to empathise, not suggest you don't deserve the hours.