The logic goes something like this:
In order to minimise the spread of the virus, schools need to close and people need to self-isolate.
However, the Great British public has a tendency to not trust the government and think that they can manage a situation by doing what they think rather than what they are told.
A significant minority of people will think medical advice only applies to everyone else and take measures that will actually increase the spread of the virus, like going to their second homes and leisure locations, and increase the risks to the more vulnerable like stockpiling.
Another significant minority will complain because they only value schools as somewhere convenient to pack their children off to for 6-10 hours a day. They will try to replicate the school childcare model by putting groups of children together. This is worse than keeping children together in schools where the contact groups are local to each other, identifiable through registration lists, and the children protected through robust safeguarding procedures.
So the stated plan is to keep the schools open.
Once the majority are saying that the schools need to close in order to reduce the virus spread, and are showing that they are likely to comply with the restrictions and inconvenience this brings, the schools will then close.
The government is seen to be listening to the people and can go further in the measures they take as they then have the majority with them.