"At small, select, universities this is perfectly possible. To some extents, these students have chosen this route with their friends."
Bubbles I don't think you are right, and not sure where your information comes from. Able pupils from super-selectives, and don't forget that top London day schools will send close to 50% of any cohort to Oxbridge, tend to opt for London colleges like Imperial. LSE and UCL as a second choice. Not because of friends but because the courses are seen as rigorous.
The first term is then different. DS saw a lot of school friends, and spent some time hanging out in Imperial. DD, in contrast, felt excluded slightly from her school friendship group because almost all ended up at Imperial and she was not around. But it changes, certainly from our experience, as DC became involved in their courses. Not dissimilar from the way many will use their second year to move on from the friends made in their first.
That said I don't know if this also applies to humanities students who (stereotypically) tended to have broader social circles at school, and who then don't have as many contact hours at University. Social life is very different if you don't have 9.00am labs.
DD's University experience is different. There is only one other person from her school year there and most of her friends, both from course and sport, have state backgrounds. There is apparently a big group of ex-boarders who tend to live in certain halls and perhaps take certain courses, and have a reputation for sticking together, but she has not really come across them. Perhaps the science thing again. Her course is so heavily loaded and the schedule so different, that she has had to make an effort to meet non medics, by playing for a University team rather than a medic team. Even then they thought she was antisocial as she dipped out of the post match socials early, till she explained she had to be up at 7.30 the following day to get to a remote placement. I suspect that one advantage of Imperial is that everyone is on the same sort of timetable.