Sorry, but I doubt it will work.
When you connect to any website, your IP address is revealed to that site. (It has to be, otherwise the site wouldn't know where to send data to). IP addresses are allocated on a 'by country' basis, so the US iTunes server will know that you're in the UK and prevent you from using their service. (This is just the same as when someone overseas tries to listen to a football commentary on a BBC website. They can't do so because the BBC doesn't have international broadcasting rights, so the BBC server won't send them the audio signals).
You might be able to get round the 'electronic barrier' by installing software which lets you use a US proxy server but I wouldn't want to put money on it!
Chris