Just curious... will there be anything connected to the balloons? Sometimes schools will hang notes, etc., from balloons so that when they're found the finder can let the students know where the balloons ended-up. If so, a call may be in order.
And I don't know how it would be received in the UK, but where I'm at a courtesy call to the local ATC unit, regardless of the rules for such a small number of balloons, would not go unappreciated...
Dave