He'd have to say what he suspects you of and why. He can't just say "I'm a policeman and I think you're up to no good, tell me your name".
There are rules and the police, like the rest of us, have to play by them.
No, they aren't. There is no statutory provision requiring you to provide a police officer your details unless they suspect you have committed an offence.
Of course it matters.
Whilst the off-duty civilian employee had every right to ask him who he was and what he was doing, he had every right to tell her to mind her own business. The police who got involved should have know how section 43 should be applied with regard to "reasonable grounds to...
Police take a rather heavy handed approach and the Chief Constable defends them.
http://www.brightonandhovenews.org/2017/05/04/sussex-police-detain-bbc-cameraman-under-anti-terror-law-for-taking-photo-of-hove-town-hall/