That officer was talking shit, read the legal thread for more info.
Doesn't mean he wouldn't have wasted an hour or so of your time though.
He actually had no juristiction or authority, its a civil matter and the police are not there to judge civil matters, who was he acting on behalf of? theres no criminal activity going on, he had no reason to stop. You were 100% correct in your understanding of the law, he wasnt.
If confronted I say I'm just looking round, at a last resort ill say I'm taking pictures as its my hobby but I don't offer that up easily. After that the usual response from me is why? do you expect me of committing a crime. If pushed ill say im looking around the place to buy it, anything on the market comes with permission from the estate agents (it normally says anyone with a copy of the details is allowed to look round). If they ask who gave permission i go the route of asking if they suspect me of a crime and what that crime is and what evidence they have.