It was actually one of the sales assistant that told me the first time. If you're female and hesitate at Boots for even a second, someone will approach and try and sell you the latest wonder cream from their own range. As I was flying out to the far east she pointed out i would save even more because of vat exemption. Since then I stock up on certain things as long as I'm checking in buggage on the way back.

Note there are two branches of Boots so if they are out of stock in one, it's worth trying the other.