Unfortunately the UK equivalent of ACLU,
Liberty (the National Council for Civil Liberties) doesn't take such an interest as ACLU does on this issue.
Perhaps because the UK lacks any formal written constitution guaranteeing the rights of it's citizens, it's rather easier to say what
isn't permitted rather than what
is.
There are plenty of sites which gives you a list of what you have a "right" to do (for example,
photographersrights.org.uk) - but I'm not sure I'm confident that they are sufficiently authoritative to for me to trust completely.
The other end of the scale are the "lawyers' advice" type sites (e.g.
http://www.sirimo.co.uk/2009/05/14/uk-photographers-rights-v2/) which are pretty clear but really only tell you what you might be nicked for.
Fortunately, a a few years ago the Commissioner of the Met Police issued an advisory letter to the force making clear how photography and photographers should be treated (i.e. not as a criminal or potentially criminal activity which should be stamped out), and apparently carrying a copy of that might be handy to show any officers interested in one's activities, even from forces other than the Met.
I suspect many forces will have had a bit of a wake-up in the past year after the case of an officer from the Gloucester force who threatened, harrassed and was generally abusive to an amateur photographer. The photographer filmed the entire incident on his mobile and the officer was suspended and subject to a disciplinary hearing.
Photographer 'arrest' threat: Policeman guilty of misconduct - Amateur Photographer