Straight in
Three years, four with a placement, entered directly on your school-leaving qualification. This is the default route and the one every published entry requirement describes. If your grades sit inside the typical range for the course, this is your route and there is nothing to think about.
Foundation year
A preparatory year attached to the front of the degree, designed for students whose qualifications do not yet meet the direct-entry requirement, or whose subject background does not match. Four years in total instead of three. It costs an additional year of tuition and living costs, which is the real trade-off and is worth calculating rather than hand-waving.
It is not a consolation prize. For a student changing subject direction, or coming from a school system that does not map cleanly onto A-levels, a foundation year is frequently the sensible route rather than the fallback.
Top-up
The final year of a bachelor's degree, for students who already hold an HND, HNC or a two-year diploma in a related subject. One year, and you graduate with the full degree. It is the least well known of the three and the most valuable to the students it applies to — a diploma already completed at home can be worth two of the three years.
The catch is subject alignment: a top-up expects you to arrive with the first two years' content already covered, so the diploma has to be close to the degree.
How to work out which applies to you
Start from the qualification in your hand rather than from the degree you want. School-leaving qualification, grades inside the range: straight in. School-leaving qualification, grades below the range or the wrong subjects: foundation. A completed HND, HNC or two-year diploma in the subject: top-up. The eligibility check will put your grades against what each university typically asks for, which is the fastest way to find out which of the three conversations you are actually having.