I've got an idea: pre-printed excuses!
"In line with the model used in related laws, the offence itself is "strict liability": it is the gathering of information that will be deemed to be the offence, and a defence that the person had a "reasonable excuse for their action" is only allowed after the offence has been charged."
While the "reasonable excuse" only counts "after the offence has been charged", I'm sure the police wouldn't mind knowing, in advance, what that excuse is likely to be.
So, how about pre-printed excuses?
When the police officer first intervenes, just say, "Here's the excuse I'll use in court if you try doing me for taking photos." Hand the officer the sheet of paper concisely stating the planned excuse. Then the police officer can see what the court will be told by your defence team if it ever goes to trial.
Of course, plod might decide to ignore the pre-printed excuse. "Okay," you say, "have it your way, and leave yourself in the dark." Even if plod persists in keeping themself at a disadvantage, you've still got that evidence of your excuse for use in your defence during your trial. And when, during your interrogation down the local nick (or in Paddington Green police station, because you're a terror suspect), they ask you why you were taking photos, just refer them to the pre-printed excuse as your answer.
But what should such a pre-printed excuse say? A lawyer's help in drafting it would probably be a good idea. But one idea, for use when photographing demonstrations and protests, is the following:-
"My reasonable excuse for taking these photos is for use as evidence in the event of police officers abusing their powers against a lawful demonstration."
And at the bottom of the pre-printed excuse, be sure to have something like the following, in big print and bold:-
"IF THE POLICE AREN'T DOING ANYTHING WRONG, THEY HAVE NOTHING TO HIDE, AND NOTHING TO FEAR."