Shipping back to UK is fairly easy, you already have the address but will also need a CN22 custom form. These can be obtained at the post office or alternatively fill it in online and print.
http://webapps.usps.com/customsforms/#
It's nothing more than a delivery note which declares the contents and value and that you are responsible. remember to sign it.
Pack up and send, are Zavvi asking you to pay for the return?
Measure the box and the weight. dent input into USPS website for a quote.
http://postcalc.usps.gov/