Now, I don't know how it works in the UK, but here in the USA, a photographer is well within his rights to photograph whatever he wants, with only a few exceptions. If you are shooting from a public place, you can shoot anything you want, except a person in a situation wherein they have an expectation of privacy. This can be a tricky bit of semantics, but I expect that most people possessing some common-sense will figure it out. Don't try to shoot somebody in a bathroom or dressing room. You can shoot them on the beach, but you're probably just better off not.
You may find yourself in a spot of hot water if you attempt to photograph anything related to the military, the federal government, or possibly local law-enforcement. And I'm told you'd better beware in NYC with landmarks and public transportation.
Also, just because you may, ultimately, have the law on your side regarding most of your photographic endeavors, that doesn't mean you won't get stopped by the police, questioned, and possibly even detained or arrested. Often, the police think they can legally stop you for photographing even when they cannot/shouldn't. You may find yourself being in the right all the way to a jail cell.
Now, when I say that you have a right to photograph just about anything you want, you must understand that you might easily not have the same sort of latitude when it comes to the displaying or publishing of your images. If the subject matter is private property or a copyrighted work, you may not be able to do anything with the image without permission. If the subject is a person, you are at some liberty with editorial or artistic uses, but any commercial use will require permission unless you really can't discern the person's identity.
I've never been stopped by police when photographing, but I've been stopped on several occasions by security guards, random do-gooders, and a rare competitor (especially when they are the "official photographer" at an event and I am shooting for fun/art/practice). I find these moments annoying and I really can't even imagine the amplification that would be part of an intimidation by police. They do have guns, after all. I prefer all shooting to be done with a camera.
For more information, check out Bert P. Krages II's Photographer's Rights pdf document. You may find it very enlightening!