Pumpkin - I really do hope your DS hears soon from Imperial and / or UCL. I just wanted to say, stunning though Cambridge is, I’m sorry but give me South Ken any day of the week
I kind of wish DS had been STEM-inclined because what a privilege it would be to have the opportunity to go to Imperial. I don’t know if you’re too familiar with it round there, but we’re generally lurking around there several times a week and you see the Imperial students. They have Hyde Park right there and they all hang out. Exhibition Road is pedestrianised, as is much of South Ken. At Xmas there is a lovely atmosphere with the ice rink in the corner of the Natural History Museum. It’s like a village really - you just happen to have things like the V&A thrown in! So many cafes and stuff to discover, or just hang out and watch the world go by. Did you say the main accommodation is in Acton? Don’t worry about that because South Ken tube is right there and it’s only 5 stops or so to Acton (on the Piccadilly Line). This will be why and that’s an easy commute. DS has friends in South Ken and Acton and they’ve all been backwards and forwards on that section of the Piccadilly Line since they were about 11. It’s no problem.
As for where London students live in the second year I’m also wondering this. DS could potentially move home, but I don’t think that would be ideal.
Regarding UCL, we did go up there just before Xmas. They wouldn’t let us into the campus, but I was amazed at how large the campus is, tucked away there. How did I not know this? It’s like an oasis and also quite an imposing sight from the gates. My youngest said, “Is that the White House?” 
If you wander through the UCL campus there is something near Russell Square called the Garden Halls. It’s an intercollegiate hall, but will be mostly UCL I suspect. It’s like a hotel. Landscaped gardens, tennis courts, allsorts.
I am looking into potential accommodation at the moment too.