There isn't a single, readily available figure for the exact number of children attending school in Britain. The number fluctuates and depends on how "school" is defined (including things like preschool, nursery, primary, secondary, and special education). Data is collected and reported by different government agencies in different ways.
To find the most up-to-date information, you would need to consult official UK government statistics, likely from the Department for Education (DfE). Their website will have detailed breakdowns by age group, type of school, and region.