In the UK there isn't any invasion of privacy. Street photography is perfectly legal unless you are harassing someone.
Maybe, maybe not. Depends on location/country and jurisprudence.
To give you an idea, and maybe that is different in the UK, in the Netherlands (with one of the more liberal Copyright systems, if not superseded by European law), it is as a general principle permitted to take pictures of scenes in public space, including pictures of people. There are limitations, usually around copyrighted objects in the scene, but people are not excluded from the general principle.
However, there is also a portrait right section in the Copyright Act, which describes that publication of portraits taken of a person might be prohibited. There are several situations which legally do allow the use of such a portrait, but one of the possible illegal uses is if the portrait conflicts with the personal interests of the person(s) shown in the image.
Those interests could be anything, and it's again up to the judge (the courts) to decide if those interests are at stake. Therefore, the general rule is, since you cannot know if you are violating the interests of the person in the image by publishing it without permission, it is wise to not push your luck or ask for permission. For example, maybe someone is (accidentally) seen together with someone of questionable reputation, or in a location where he/she should not have been at the time.
The very clear explanation of the Dutch Portrait-right section of the Copyright law can be found
here in a Google translation, so it will read somewhat crooked here and there, but you'll get the gist of it.
People tend to forget that rights also come with obligations, i.e. to use those rights with proper care and consideration for others.
Cheers,
Bart