From the BBC's guidelines for filming people in public places:
http://www.bbc.co.uk/editorialguidelines/guidelines/privacy/
Legitimate Expectations of Privacy
An individual's legitimate expectation of privacy is qualified by location and the nature of the information and behaviour, and the extent to which the information is already in the public domain. People in the public eye may, in some circumstances, have a lower legitimate expectation of privacy.
Location: People in public places or in semi-public places cannot expect the same degree of privacy as in their own homes or other sensitive locations. (A semi-public place is somewhere which, though private property, gives the public general access, such as an airport, station or shopping mall.)
However, location must be considered in conjunction with the activity. There may be circumstances where people can reasonably expect privacy even in a public or semi-public space, particularly when the activity or information being revealed is inherently private. For example, there may be a greater expectation of privacy when someone is in a public or semi-public place but receiving medical treatment.
There may also be occasions when someone in a location not usually open to the public is engaged in an activity where they have a low expectation of privacy, for example a sales pitch or giving public information. We do not, though, normally reveal information which discloses the precise location of a person's home or family without their consent, unless it is editorially justified.
Behaviour: There is less entitlement to privacy where an individual's behaviour is criminal or seriously anti-social.