Universities are independent, self-governing bodies which have the power to award their own degrees. Colleges of higher education do not normally have degree awarding powers. A university will approve a college's courses. Colleges tend to be smaller institutions. However the institutions will assess applications from international students on an individual basis.
Most students apply while they are still at school.
The IB is an accepted qualification for entry to UK universities and colleges, but contact the institution to make sure of the IB points they require. Universities and colleges of higher education in the UK do not normally have entrance exams, except for Oxford and Cambridge. In some cases you might get asked to do a special assignment, e.g., write an essay. If you are applying for an art or design course, you will normally be expected to produce a portfolio of your work. For music and drama courses you might have to audition.