There was a Tweet from President Trump, about the UK's spy agency spying on him in the 2016 elections. Well, governments of other countries have been spying on each other's citizens for years. So, if the United States wants to legally spy on their own citizens one way they can do it, is "allow" the UK government to spy on Americans and just give that information to the US government. Like wise if the UK's system doesn't allow or restrict how much spying can the UK government does on its citizens they would just get the United States to do it to the UK's citizens and give the information to the UK government. They don't "officially" say they spy on each other's citizens but there are evidence their that they do, aka, Edward Snowden's leaking files about the NSA spying on Americans.