As this is a BBC show (CBBC to be exact), the Clarke family is obviously English and they reside in the U.K. Yet in @ the 26:41 min mark, we see an outside shot of the Clarke residence with a close-up of their mail box (complete with the red flag in the "up" position). The Clarke's "English" mailbox is clearly stamped with the following,
"CLARKE
U.S. MAIL
Approved by the Postmaster General"