Did the police catch him in the commission of a crime? If not then, yes, they would need a judge to sign a warrant. Seeing as he was released without charges it seems the police grossly overstepped their authority.

Edit: Let me caveat because I fired fast and loose with this reply. This is from an American perspective. I am not a UK lawyer so this could very well be perfectly legal. Though I do view it as "bad" and would oppose the introduction of these norms in America.