If this is the case and such monitoring was negotiated through a treaty with the Brits, this would be an abuse of federal government power to negotiate treaties imo. Not only had Thomas Jefferson warned about such abuses of treaty power, but the Supreme Court had officially clarified that such abuse is unconstitutional.
"In giving to the President and Senate a power to make treaties, the Constitution meant only to authorize them to carry into effect, by way of treaty, any powers they might constitutionally exercise." --Thomas Jefferson: The Anas, 1793."Surely the President and Senate cannot do by treaty what the whole government is interdicted from doing in any way." --Thomas Jefferson: Parliamentary Manual, 1812.
"2. Insofar as Art. 2(11) of the Uniform Code of Military Justice provides for the military trial of civilian dependents accompanying the armed forces in foreign countries, it cannot be sustained as legislation which is "necessary and proper" to carry out obligations of the United States under international agreements made with those countries, since no agreement with a foreign nation can confer on Congress or any other branch of the Government power which is free from the restraints of the Constitution (emphasis added)." --Reid v. Covert, 1956.
He says that within about a week after September 11, 2001, NSA "decided to start spying on the U.S. domestically, on all U.S. citizens they could get". He says the data center in Utah has the capacity (five zettabytes) to store all worldwide communication for 100 years.
I rather doubt it is anything so formal and as well documented as a treaty, it's just an understanding...