Arewedone, UCL is very worth looking at as an alternative across a range of subjects.
There are advantages in a University which is not solely focussed on science or social science, and it has a strong reputation pretty much across the board (it is comes 8th in the research orientated QS world rankings). It also offers some interesting degrees, like Liberal Arts with scope to have a science or social science major, PPE, a very well regarded management course, and an economics course that is similar in status to LSE/Cambridge. The big big attraction of UCL is the ability to have a year abroad including at some very good US Universities. They have been building a lot of accommodation and, I understand, a second campus at Stratford in the Olympic Park (great if/when Cross Rail ever opens).
The other good thing about London is that just about everyone passes through at some point. We went to a talk by Joseph Stiglitz last week, with plenty of students in the audience, the LSE has a really good series of public lectures, international banks send along their Chief Economists to give quasi recruitment breakfast talks, and so on.
The approach your son's school is using sounds strange. DS' school sent coach loads to Oxbridge which meant that at least equal numbers were rejected. It was also recognised that some subjects (NatSci, Law, economics/E&M/PPE) were really competitive and decisions were made on fine margins. Pupils/parents were therefore encouraged to focus on a wider range of courses/Universities, or plan for a gap year and reapplication. London was generally seen as academically on the same level (Imperial, say, offers the advantage of greater specialisation) but obviously with a different student experience, so by far the most commonly chosen UK alternative.
Do PM if you have any specific questions, which I will happily pass onto my son.