I don't think what you are suggesting is ok. At all.
However, there's a few possible outcomes. The school will look at their policies and decide they aren't going to change them. Then you can either change schools (which will have the same or similar policies) or pay for after school care.
You could legally challenge it but I suspect that would be costly and take time. You'll have to pay for after school care in the meantime.
The school decides to relent and allow your children to walk home alone to an empty house. Then any incident and injury no matter how accidental will be treated more seriously since your children were left alone.
That's assuming it's only ever half an hour and you don't get stuck in traffic. Have you thought about what should happen if you are in an accident? How long would it be before someone noticed your children were alone?
Social services won't be interested now because they aren't in danger now. That will change if something happens and the fact that they were alone will be taken into account.
As some have suggested just leaving them at school, if the OP is 30 minutes late the police won't be called. That only happens if there school cannot contact a parent at all or literally everyone is going home for the night. More likely, and what many schools do, is have a policy of late pick up being charged a care fee or placed in an after-school club which will be payable. So essentially paying for after-school care.