Spr1ngchick, a few more thoughts. All children are different. DS was very keen to leave home so lived in hall in his first year. I think he needed the sense he had left school and home. He then had the chance to take up a spare room in an established flat in his second year. Its not clear what will happen in his third year, but it is useful that he has moving home as a Plan B. For him getting away was useful and if he came home now it would be as an adult, with him expecting to pull his weight with housework, laundry etc. He has also managed to land a three month summer internship outside London, which hopefully will help widen his geographical horizons.
A few of his friends have ended up moving home for their third years. The work load is heavy, getting a good final result is so important, and they find they get more done in their home bedrooms, with fewer distractions.
DD recently had an interview with another London University which would not have offered her accomodation. She was rejected, but had she ended up going there I think she would have been quite happy staying at home. She is quite sociable so though it might have taken her a bit longer to get going, she would have found her group and given plenty of others would either have been living at home or in some far flung hall, she would not have been at a disadvantage. Our concern would have been more about how we altered her status within the family unit. She will get a fair amount of leeway through the Yr13 summer of exams, but we would not want this to continue. I guess we would have looked at encouraging her to spent the summer somewhere else, perhaps some sort of residential job, perhaps learning to drive, decorating her bedroom etc, and would have consiously tried to treat her as a responsible adult. A couple of positives would have been that she could have continued with her volunteering, and with her sports club, which she felt would have given her a nice balance/grounding away from student life.
Oddly accommodation seems more of an issue where she is going, as she likes her sleep and will have regular morning lectures. The University has a bit of a reputation for being party-town. Staying at home is definately a lesser evil than being in a student flat full of late-night clubbers.
One thing mentioned by a previous poster is that Halls often have spare places after Christmas when overseas students, doing a term abroad, leave. If your DS wanted a break from home, he might be better off waiting till then. He would save the first term's accommodation costs and would know where he wanted to live.
London is different, obviously, but if your DS is not hankering after the full-blown campus University experience, there will be lots of positives. Student London, whilst not geographically separate, will be a different place from the London he already knows. And if he works reasonably hard he should emerge with a well-respected degree, an international outlook, and a certain amount of city-smart, which should all aid his future employability. Plus less debt.