They offer you a contact number on their site
Outside the United States and Canada
+1 813 979 6845

Using the cheap dial thru services, you can call the U.S for less than calling 0870

However.......
Subscription enquiries to:
National Geographic Kids, PO Box 326, Sittingbourne, Kent, ME9 8DF

This address is the home of Galleon ( www.galleon.co.uk ) mentioned before, somewhere on this site.
They handle the subscriptions for many magazines, and have a land number starting with 01795 4147** the last 2 digits depend upon the magazine. National Geographic obviously want to take more money from you.

Try 01795 414749 for BBC Music, and see if they'll help.