When I lived in the U.K. the same thing happened to me every time I ordered from Brent......it's just one of those things with the import duty rules. It can be avoided (sometimes) if the packet is marked as a 'gift' but you can still come unstuck if the Customs people look into it.
I now live in Spain and it's worse here. Import duty, I.V.A. (similar to V.A.T.) plus a Post Office 'handling charge' !!!!!!