If USTVNOW is available in the UK channel store you will be able to watch the US networks for no charge on your TV using a Roku. If you want all the typical US cable channels including HBO you will have to subscribe to one of USTVNOW's paid subscription plans.
If you are in the UK now you can use your PC and watch American TV using USTVNOW and see if this is what you want.
That is ONLY true if you meet the following conditions: (From the USTVnow site)
I am an American citizen (by birth or naturalization) and hold a United States passport or ‘Green Card’ for permanent residence. I understand that USTVNow’s Internet content is available, at this time, only to US citizens and residents while they are in foreign countries. I promise and warrant that my use of USTVNow is and will be consistent with this limitation.
If your status and/or use of USTVnow is not within those guidelines you will be in violation of the terms of service and, possibly, US law.