“I don't understand why councils need to find spaces for all children.If parents opt out of the state system they are relinquishing their school place and surely must know they are running the risk of not getting what they want if they decide to switch back again.”
That is not how the social contract works. Every child has a right to a state education.
Besides, every person also has a right to NHS treatment, even if they opted, temporarily, to pay for private healthcare.
Finally, if every private school family left the country tomorrow, the country would probably be bankrupt as there would be an immediate and significant loss of tax income. It is important to remember that.
So the state should simply find state school places for all children, within the time limits set out in our existing laws and pay for transport, if required.