Just ordered this from Amazon.com (US site) for £13.35 including delivery. As it costs less than £18 you will not get charged duty or VAT by UK Customs so £13.35 is the actual price.
There are many other HD-DVD bargains as well. Just make sure the items you order do not go above £18 (excluding delivery) to avaoid customs. Cheapest UK price I can find for Transformers (in-stock) is £22.
Dreamworks Transformers - HD-DVD
From director Michael Bayand executive producer Steven Spielberg comes a thrilling battle between the heroic Autobots(R) andthe evil Decepticons(R). When their epic strugglecomes to Earth, all that stands between the Decepticons(R) and ultimate power is a clue held by young Sam Witwicky (Shia LaBeouf). Unaware that he ismankind's last chance for survival, Sam and Bumblebee, his robot disguised as a car, are in a heart-pounding race against an enemy unlike anything anyone has seen before. It's the incredible, breath-taking film spectacular that USA Today says "will appeal to the kid in all of us.".