The 1st and 2nd rapes both happened in 'the field'.
GrumpyJim and TaxiClive are both in the most advantageous jobs, as breakdown recovery service and cabbie, for hearing [and/ or tipping off 3rd party attacker(s?)] about times and whereabouts of any lone, vulnerable women.
"Seeing as Miller saw the video on her son's phone I think we can assume it wasn't Trish or anyone underage."
True, if she'd seen close-up faces, NotJanine (and I agree is definitely not pre-adolescent child porn, thank God) but 14/ 15 yr olds could be passed off as legal, late teens, I suppose.
The amateur filmer(s?) could purposely have angled camera to disguise identity in the eventual footage? Or pixelated? (Yet threatened those filmed with releasing an uncut version, if they didn't keep quiet?).
In Miller's DS's clip, the female didn't sound at all gagged/ muffled, so not Trish's rape. (Or either of the other 2 women). Doesn't mean it was necessarily consensual, though, sadly.
Can well imagine ArrogantTwineBoy (with help from IceCreamGirlfriend, perhaps?) grooming the underage teen girls with attention, expensive presents etc., convincing them to send compromising selfies, then blackmailing them to go further: pimping them out to other Broadchurch ring members in RL? Football team players? On film?
Vile 
Hardy's DD refused to take part, was exposed as consequence, and had to break silence about her picture to her DF. Who better hurry up and make a connection.