The easiest definition of college and uni in the UK is that college is for further education and uni is for higher education.
so u would leave school and go to a colege to learn a practical skill .. then go to uni to enhance the status of that skill by getting an academic qualification.
on the other hand, you could do further education in school (A level education) and then go straight to uni and study for another 3 yrs and get a degree.
This option would also teach you how to be arrogant and useless at practical skill such as tying shoelaces - whereas the college route would teach u a skill u could use when u find it impossible to get employment through ur academic qualifications alone.
A uni also gives u the option of doing a post grad course for people who have not learnt enough practical skills to be let out into public places without supervision. This couse generally would make u a recognised expert in ur subject and give u a phd - unfortunatly, it will not teach u to tie ur own shoelaces.