Generally, there is no legal obligation to pay your staff if they are late in or do not arrive to work due to adverse weather conditions. They will have to take unpaid or annual leave. Having said that, you should check what is written in your employees’ contracts of employment, as you may have made some concessions.
If your employee is unable to get to work because they need to look after their children as their school is closed, this may qualify as Time Off for Dependents. An employee is entitled to as much of this unpaid time off as a tribunal would see fit.
Ultimately, you should ensure your policies are clear and applied as fairly as possible.