I'm not sure how it works in the UK, nor am I really sure how it works in my own country (Canada), as the lawyers like to debate these issues to the end of time. That said, my understanding of it is this. Anyone can film anything in a public forum. Where you may be sued is if you use that public footage in a manner that could be construed as defamation of character. For example, if I'm making a video about prostitution and I videotape women waiting for the bus or teenage males cruising in their cars on main street, and I use that footage as b-roll in my film, but in such a way that those persons are depicted as prostitutes or 'john's', it would be pretty good grounds for a defamation of character lawsuit laid against me. If I was actually filming prostitutes and 'john's' cruising around the red light area of my city, and I disguised their faces in final production, I'd be minimizing the chances of a lawsuit, as I've eliminated a great deal of possibility for someone's character to be defamated.
At the end of the day however, anyone can sue anyone for anything. All you have to do is file a writ in a civil court. So, there is no 100% protection from a law suit. What you can protect yourself from is the credibility of the plaintiff's lawsuit.
If you are filming people in a private setting, such as a library, you will likely need permission from the library to do so. The library will then probably put up a poster that warns people of the shooting and gives them the option to inform you if they don't want to be captured in your film.
As for the 16-year-olds and their consent. I think it depends on two factors; the age of majority in the UK, and wheter or not you have something like an Infants Act in the UK. The Infants Act in Canada allows counsellors to provide their services to children under the age of majority, without consent from their parents, provided that the counsellor considers the child to be old enough to fully understand the consequences of such services. Maybe the UK has a law like that but pertaining to the rights of a child to access any kind of service.
Those are just my thoughts