I don't know about the UK, in the US you can record (or film) anything you want in a public place. There is NO expectation of privacy when you're in public.
I think the UK was the same, most places are - you can't have any expectation of privacy when you're in public. If you did, the paparazzi / tabloid newspaper industry of photographing celebs would be sued to oblivion.
But that doesn't mean filming someone with the sole purpose of embarrassing them or presenting them in a bad light, especially if it's done by selective editing and inclusion of clips to present a false impression, is morally justified.
i.e. if you interview 10 JWs and 7 can't explain their beliefs, that's a story. If you interview 10 and only 1 can't but you play only that clip (and don't bother to find out if they are maybe a new convert) then you're a dishonest lying piece of shit.
Lies by omission is common for those pushing a specific agenda vs actually fairly reporting on reality.