“””US law cannot compel someone in Britain to be treated as a citizen of the US”””
Indeed, it can and DOES.
Current example: The IRS is currently targeting “accidental Americans” who live abroad for taxes and penalties: “The IRS is making a worldwide push to squeeze money from Americans living abroad and from anyone who holds dual citizenship, whether they know it or not. It doesn’t matter if the “duals” want US status, have never set foot on US soil, or never conducted business with an American.”
More detail, here (http://mises.org/daily/5666), but you get the gist.
The U.S. CAN in fact compel someone in Britain or anywhere else in the world to be treated as a U.S. citizen even if they are completely unaware that they are one.
Likewise, the same is true of other countries’ ability to do the same to their citizens located in the U.S.
Any self-respecting foreign government would laugh at that.