Most definately yes.
What you have to do is set up another account on your PS3, but this time when it asks for region/address, just give a U.S. address [search for a Mcdonalds address in the U.S using google, I typed in Beverly Hills 90210).
Now you should have two accounts on your PS3, one UK and one US. When you log into the U.S account, go to Playstation store and it will automatically take you to the US store with the Dirt Demo.
By the way, you can sign up for a third account using a Japanese address. This will allow you to download a demo of Golf [Hot Shots 5 I think, excellent game].