Whether I like it or not, I do expect foreign governments are (at least attempting) to watch any American citizen they have an interest in. That's the way spying and espionage works, and has worked for every country that is or has been in existence. If you honestly believe the UK does not have it's own intelligence services doing the exact same thing, then you are in for a very rude shock in the future. Hell, didn't you guys come up with James Bond, who is based in part on the very real life of Ian Fleming?
While prior to this point lack of resources have prevented foreign governments from collecting data on everyone, that is simply no longer the case. I suspect random bits of my internet history is sitting in many foreign governments' systems (and many foreign citizens' internet histories are sitting in American systems). However, there is a key differences between my government having my data and some foreign government having my data. The Chinese, Russian, UK, Australian, and other governments have very little control over me, and I don't have much control over them. So while I'm not really happy about it, I'm not really all that concerned about it either.
However, my government does have some control over me, and I it. That makes us, to some degree, adversaries. As the recent revelations about the IRS illegally targeting political groups contrary to the current government, there is reason for politicians to find information about me and use it to try to disenfranchise me, either directly by finding some act I have performed that can be construed as illegal, or else behind the scenes through blackmail, manipulation, or frustration. Further, the American constitution explicitly prevents collection of information about American citizens anywhere and anyone in America regardless of citizenship status, except in very specific circumstances (usually requiring a warrant) - something other countries don't necessarily have. So there are many reasons why I am okay with your privacy being violated by my government, and mine not. In a perfect world, I'd prefer no one's privacy being violated, but I don't live in a perfect world.
Of course, it is really hypocritical for a citizen of the UK complaining about privacy violations by a foreign government, given how your government has been increasing surveillance of their own citizens with very little outcry.