I'm sorry to hear about the outcome for DT1, Coleoptera. I am not normally an advocate of gap years for people who haven't got the university they want, but in DT1's case, if he's really very keen on Durham and is likely to see all other options as unbearably inferior, I suspect he would have a higher chance of success there with his A Level grades in hand.
From what you have said in this thread, I would have two red flags when considering his narrative from an admissions point of view. The first is, as you say, his GCSEs, while very good, are not particularly great relative to his cohort. Secondly, based on those GCSEs and his school type, I would question whether we are looking at someone whose A Level predictions have been overinflated by a school that knows how to game the system.
Narratively, this gives two options with some grey areas inbetween. The first is: "We have an applicant whose ability and/or application in subjects they don't enjoy or find easy is patchy, and all the advantages his school confers haven't been able to make up for that, resulting in that spread of GCSE grades. It would be unsurprising if he dropped grades in his less favoured A Levels too and may continue to do so when faced with modules on his degree course that he similarly finds difficult or uninteresting. His ambitious school probably think he will end up with AAB but are hoping that predicting 3 As will get him an offer and he'll scrape in on results day. This is a very good student, but not an exceptional one and we don't need to take a risk"
The second narrative option is: "We have an applicant whose GCSE result are not, in context, exceptional, but they appear to be very strong in humanities subjects and they are high enough across the board to suggest he is a generally capable student. His results in his English, etc., GCSEs suggest that he does extremely well when studying subjects he enjoys or has a particular aptitude for, setting him up well for degree-level study. While his A Level predictions seem high, his trajectory of achievement in these subjects supports a high prediction. He is a decent applicant"
While taking a gap year and applying with A Level grades in hand won't fix his GCSE grades, it would remove a lot of the questions regarding the disjuncture between his GCSE grades and predicted A Level grades and the suspension of disbelief currently required of admissions tutors in the absence of hard evidence to suggest he is an exceptional student capable of achieving 3A*s or thereabouts. From the sound of it, Durham doesn't interview for English, so he needs something to nudge his narrative more towards option 2 rather than leaving option 1 as a real possibility. He's unlucky really that he's applying as ASs are being phased out, as they would probably have helped his case if he had done well in them.
This isn't to say that he doesn't have some really great options for next year, but if he isn't going to approach them and his subsequent studies and career options in a positive frame of mind and always feel that he has been forced in to an option that isn't, in his eyes, good and desirable, then it might be worth considering a gap year.