Three offers sim-only deals that are free to use abroad, including the USA. I think giffgaff should offer a Roam-Free package!
With Three's "Feel At Home" package, you can use your allowance to call and text the UK and use your data without paying a penny more. There's no extra cost for calls back home, no extra cost for texts back home, no extra cost for data. More info: http://www.three.co.uk/Discover/Phones/Feel_At_Home?intid=3mainfahrbn1501
Why can't giffgaff offer a similar package? It's a great deal!