I was no clear enough in my previous post - I was talking about research fellowships that are open to you after a PhD! Oxon, Cantab, Harvard, Stanford etc are then still interested in your undergrad scores, but few other places are. I don't know that Oxon etc care that greatly about your first being 70, 75 or 80 - they weren't interested in details when I did a postgraduate qualification there, but I already had qualifications coming out of my ears that supplanted my undergraduate degree. Perhaps one of their admissions reps would be able to clarify that point.
I basically treated my degree like a full-time job Monday to Thursday. I did not work Fridays at all until the final weeks of my PhD writeup - I effectively went 7 years without bothering myself to work of a Friday. I am a voracious reader, and I can take things in quickly - that was and is my strength, and it is that which helped the most.
I also boxed clever and worked smart, so did modules that I knew I could blow out of the water etc.
But with all this said, whilst I understand (and endorse) wanting a first class honours degree, I don't think that you should put too much pressure on yourself to get over 80. It wasn't by design that I managed it (I wasn't sure it was possible in my discipline), just a happy coincidence because of an exceptional final year.