Could England, say, switch to the Euro whilst the remaining part of the U.K. stay with the Pound? If it was a sovereign  nation it could. As it isn't, this is an impossibility. Does england vote in any world governing bodies like, oh, I don't know, the United Nations? No, of course not. It would be like Utah decidng to go it alone and design their own money. England may be a former country, but is certainly not one any more.