We must have one of the most comprehensive welfare systems in the country, with the student union, individual JCRs and the university all providing comprehensive welfare facilities. It's not that surprising - lots of the "cream of the crop" are screwed up - why have many worked so hard all their lives to get the best grades rather than settle for 2nd best? There's increased workload pressure, competition etc here. A lot of depression here's caused by people thinking too much about their life - existence, contentment etc.
However I wouldn't say alcohol & drugs problems are that prevalent - the drugs scene is very underground and largely a social pastime of the richer kids, though relatively few have addiction problems.
As for the intitial question, it's not enough to make people "tip over the edge". A fair few take a year out to sort themselves out, but suicide is almost non-existent.