I think that the pre-school should put out a notice as a matter of course, but don't think it is an ofsted requirement. I don't think the pre-school can force the child to stay away either, until the headlice have been eradicated.
I know that my childrens school put out notices but they cannot force the parents to keep their child away from school until the headlice have been elimated. Nor can the force the child to be treated !!!
DD1's school does it and so does DD2's pre-school. It's more best practise that a requirement. IME Some parents will quite happily take their child home and de-louse immediately and other don't give a toss. Wehad one parent who did nothing for months with her children's heads. Despite sensitive assistance by the pre-school we had to pass it on to the HVs to deal with.
It's only fair on the other parents and kids to just advise that head lice are doing the rounds and maybe encourage everyone to have a 'de-lousing' weekend so at least they will all be clear the following week.# As long as everyone joins in.