The colleges for 16-18 year olds in the UK aren't universities you're thinking of. College at 16-18 here is for getting A levels (similar in difficulty to the more academic US high school courses, probably), not for degrees. Some normal secondary schools have their own facilities for this though.

We still go to university at 18, anyway. Difference between here and America is that we just pick one subject (or do a joint degree) and do that, instead of still having courses from multiple areas like humanities and sciences like in school.