non plus, there are two kind of degrees:
Ordinary
Honours (Hons)
An ordinary degree means you pass or you don't. You don't fail a degree, you just don't get it if you don't pass. They'll be a pass mark (say 40%) if you get above it you go the degree... the BA for example. There is no high pass, mid pass or low pass, no A*, A, B, C etc. You pass or you don't get it. Like a driving test if you will.
Now an honours degree is technically a higher standard of degree which is graded. 70% is usually a first class. 60% an upper second class (2i), 50% a lower second class (2ii) and 40% a 3rd. Technically if someone got below 40% they could still get the degree but it wouldn't have honours with it. Although in practist if you don't get 40% you usually just don't get the degree.
Usually what honours actually referes to is a degree worth having. if anyone could take your degree it wouldn't be worth anything would it? An honours degree usually suggests some level of appitude or skill toward the subject matter is nessessary...which with science it certainly is. Not everyone is capable mentally of comprehending the way the world around them works, in just the same way that not everyone can sing, or draw or do a backflip.