Mmmm--I get the same e-mail:

Unfortunately, due to the Working Advantage policies, we are unable to process any orders with a credit card billing address that is not in the United States - this includes all Working Advantage products, including the Starwood Points. Because your credit card billing address is international, your order has been canceled.

When I placed the order I used my US headquarters as a mailing address and my UK address (with US state) as my billing address. They must have run a check on the Nationwide credit card and found out that it was a UK card.

Oh well. Didn't really have the money to spend on it anyway :|