Sign into the App Store using a different account
It can be useful to have a UK and a US account for the App Store, as this gives you access to apps that not available in your country yet etc
To sign in using a different account
Go to Settings,
Then Store and Sign Out.
Now you can log in again using your different account.
To get a US iTunes account the best way is to open the iTunes Store on a PC or Mac and change the country at the bottom of the screen to US
Then buy an app that is free and set up a new account however you will need a US address for this