Hi, OP -
Those are extraordinary results; congratulations to your DS.
I will try to consider the points you and he raise, then discuss the elephant in the room.
Only your DS can weigh the inconvenience of the commute. If the transport strikes continue and worsen, what implications will that have? The difficulty of the course at I and the resulting skewed work-life balance is the flip side of its superior reputation. Again, only he can weigh it. If he tends to react badly to pressure - and his grades do not suggest this - that would in my mind be a point in favour of UCL.
Having taught in STEM at some excellent American universities, I think the preference of able British students for less coursework is a big mistake. It lets you slide. No matter how able you are, you will learn more if you put in a consistent effort across the term than if you cram your learning into a few intense weeks because nothing more is required of you. Coursework helps everyone, even the brightest, keep up and reinforces their learning. We do not assign it for our own pleasure.
I sympathise with able students who have carried a group in Sixth Form and worry about doing so at uni. TTTT it can happen, but there should be mechanisms to prevent it. Also, group work is going to be essential for the career of your DS and similar dynamics will exist. IMO it is better that he should learn to cope with them now rather than later.
I agree that a First from UCL is a bit more meaningful than a 2.1 from I. If DS is doing a MEng degree and considering the possibility of a PhD, a First is important. Otherwise, probably less so.
Is he interested in a Nuclear specialisation because he is interested in a career in the nuclear industry? That is the only reason I see, and it really isn’t necessary. A good Mech Eng degree will open the doors to the nuclear industry.
I agree that the openness of academics to UG interactions, the environment, the vibe and what just feels right are all very important.
The elephant in the room: if I understand correctly, your DS holds an offer from UCL. To reapply to I (and four more choices) he will need to let that go. Even with these stellar results, there is no guarantee that he will receive an offer from I. This is by no means unheard of. If he doesn’t, then what?
At a minimum, I would strongly encourage you to contact the lead admissions tutor for I Mech Eng, if the balance tips that way. Lay the situation out for them clearly but concisely and do not proceed unless they are very encouraging! My personal view is that the balance possibly does tip that way, but if your DS were mine I would be encouraging him to start now at UCL. If he didn’t like it, I would encourage him to excel with a view towards transferring or even restarting at I - anything to avoid being caught without a place. He could also do a BEng at UCL followed by an MSc at I.
I will be well aware that AL grades were moderated last year. The competition is expected to be harder this year. This is a tough one and your DS sounds like the type of student and future worker the UK needs. I wish him the best of luck