WiFi only no problems at all (I have purchased 3 in the US over past two years), once you have a UK plug (if you don't have one you can get them cheap on eBay, just slide off the US plug). Can't comment on 4G model. UK iTunes account will work, as will a US account. You can use either account in either country - it just takes you to that country's app store regardless of your physical location. As far as I know, no problem with warranty - I'd suggest buying from an Apple store in the US as then there should be no problems.