I vaguely remember a story taking place in a near-future setting in which one or more of The UK's component nations left the union, precipitating the final break up of The UK. These events also resulted in the royal family leaving England for (I think) Australia. England is subsequently accepted into The United States as the 51st state. All of this takes place prior to the action of the story.
I am pretty sure that I read this story in a multi-author anthology. I think I read it within the last 15 years. I remember very little else about it.