What about non-US citizens expressing their disappointment at being so totally spied upon outside of US?
Pretty much the same as Americans (or usually their government) protesting against intrusions by, say, China or Iran. Yeah, you protest it. Yeah, you complain and accuse and do what you can to stop it. But you also do the same thing right back as soon as you have the opportunity or desire.
One of the more interesting articles I read in response to this was the threat of increased industrial and economic espionage against American companies. Such spying for financial reasons is seen as reasonable by plenty of countries worldwide, and the idea that America only spies for national security reasons is seen by those nations as a polite fiction. So stories of expansive American spying are seen as signs of economic warfare that should be responded to in kind.
What about exploiting legal loopholes like US and UK cross-spying and exchanging information despite clear intent of laws that prohibit massive warrant-less domestic spying?
Those laws still apply, in that the government couldn't take legal action based on information that is illegal for the USA to gather (no such restrictions in the UK, where gathering such information isn't so illegal, yet). They might get intelligence from the UK but couldn't act on it unless they had independent information that could hold up in court.
This sort of loophole is what has stuck the USA with GTMO inmates who can never be released. The government is convinced that they are dangerous and need to be locked up. They also are convinced based on information that could never be submitted to a court.
Information sucked up under Britain's lax privacy protections would likely be as inadmissible in American court as testimony under torture. Under a rule of law reading then minimisation procedures would be in place to select from those data troves only information that could be legally acquired and acted on.
Of course, the reverse is less true for Britain.