Hi, OP -
As a former RG STEM admissions tutor I think this is more subtle than it first appears. DC usually do best where they are happiest; however what this means can be complex.
For your DS with his high TMUA score and outstanding PGs I do think having a good peer group is going to be part of the happiness equation. Just what challenges is he hoping to find in Leeds whilst cruising mathematically? I have a real fear that in the modules requiring group work, which are becoming more common throughout STEM everywhere, he will be both frustrated and taken advantage of. I don’t think he is going to find the intellectual give and take there that is more likely in Durham and certainly to be found in Warwick.
Durham is lovely but I haven’t interacted with UGs there. Whenever I visit Warwick I think the students look happy. A family member who did UG there had a wonderful time and, incidentally, played in the university orchestra. The student music scene is excellent.
I think the reaction DS has to each place is important. I hope either Durham or (preferably, for all the reasons put forth above) Warwick will feel like the best choice when he visits again. BTW I disagree with DH that a Durham Maths degree will be more useful. Warwick Maths is that special.
DS says now that he wants to settle in Leeds but he is only 17. At that age a STEM PhD, a British husband, and life in the UK were three of the furthest things from my mind! Gently, can you persuade him to make this decision independently of his girlfriend? If the relationship is meant to be, it will survive different universities.
But if Leeds is the only place that feels right and this isn’t just fear talking, I think DS should follow his instincts.