As someone who does event photography, this is of interest:
I'd be unable to work if I had to do that. 1 wedding, 100 guests, 12 say 'no photos', how can you keep it straight in your head to comply?
In the US it's simple: In public, your fair game. You have no expectation of privacy in public. Of course, for private events, most tickets/etc have disclaimers/agreements on them that serve as a photo release. In the case of weddings it's just expected that you don't make a scene with the hired photographer out of respect for the happy couple.
Now in private, is a different thing. If you were shagging on a public beach in plain view, you have no right of privacy. (I've seen it btw) As a photographer I could have shot the nice couple all I wanted (I didn't). No invasion of privacy there as they were in public. Now, had they been in their home, I couldn't walk up to the house and press a lens against a window as they had an expectation of privacy. Even if the curtains were wide open. (also seen it).
Extreme example, but fitting.
My understanding is that in the UK the same is true, but in Germany that same couple would expect privacy in both cases.
When dealing with photography in public while I will agree asking for permission is the polite thing to do, it shouldn't be a legal requirement. One only needs to see a case of police brutality, or an accident, or a crime to understand why.
"Hello officer, could you turn a bit to the right so I can catch the highlights on your blood soaked baton better please?"
"Hello Mr. Regime Bully, could you growl a bit more evilly while you drag that woman by her hair? Say, is it ok if I take you picture, it's for the Times?"
I can't see than ending well.
Public photography has captured some of the worst highlights of things like the Arab Spring, the Kent State shootings, and thousands of events in between. Laws that would stop some guy who likes to shoot boring factory fronts, would also apply to those folks, allowing government and police to stifle free speech more. As an American that lives in a nation that claims such is important (reality is often other mind you), it's important to me to push for the defense of it world over.
My position/opinion. YMMV and all that.