This should also work for the PS3 too, but I haven't tested it.
All you need to do is go into the network settings on your 360 and change the DNS servers to the following: -
Primary: 208.122.23.22
Secondary: 208.122.23.23
This makes your console appear is if it's based in the USA and you'll get the much wider US selection of content when you launch the Netflix app. You can still sign in with your UK gamertag as normal.
Possible downside - As you're using DNS servers based in New York, this may affect your connection negatively when playing online games, but you can always change the settings back or switch between them when you need to.
How to get the US Netflix selection on a UK Xbox 360
Moderator: Sigma