If a police officer believes that you've been behaving 'in an anti-social manner' he/she has the right to demand your name and address, and it's an offence to fail to do so:
http://www.legislatio...uire-name-and-address
The definition of 'anti-social behaviour' is acting "in a manner that caused or was likely to cause harassment, alarm or distress to one or more persons not of the same household as himself"
http://www.legislatio...-and-disorder-general
Otherwise a police officer can only stop you and require you to 'account for yourself' (in relation to your actions, behaviour, presence in a particular area or anything which you might be carrying). Under those circumstances you're not required to provide your name and address.
However the police have additional powers when in a 'designated area', as defined by the Terrorism Act 2000. They include, for example, to power to stop and search anyone, without giving (or needing) a reason. There has been considerable concern expressed that the police (with Home Office backing) have 'designated' many areas of the country, and continue to renew those designations without effective challenge. For example all railway stations, and most of central London, are 'designated areas', where you can be stopped and searched (without any reason at all) at any time.
Chris