Police officers have the power to stop and search anyone under the Terrorism Act 2000 to discover whether he has in his possession anything which may constitute evidence that he is a terrorist.  They do not need a specific reason, your presence in a specific area is sufficient excuse.  Cameras may be seized and retained until it is determined whether photos have been made for terrorist purposes.

Refusal or non-cooperation will result in arrest and punishment by fine of up to £1000 and/or 6months imprisonment.

You are not obliged to give your name and address or explain what you are doing, but in practice you may be arrested on some other pretext if you do not.

The authority for these stop and searches arises under Section 44 of the Terrorism Act, which is why they've become colloquially known as S44 searches. They are highly contentious as they appear to be widely misused. You should ask police to explain whether they are looking for terrorists or trying to deter or delay you. Also make sure they issue you with a Search record. Consider making an official complaint if you feel the search was carried out wrongly or for the wrong reasons. Also fill out Liberty's search monitoring form