You also can't really relax so to speak because one slip up and a 60% and you need an 80% to make up for it. Obviously I'm speaking from experience of an engineering degree where getting a 1st is fairly impressive.
Yep...you got it in one.
To get a first, you have to put in a lot of dedication and effort. And as alx stated, you have to be consistent...unless of course you are prodigiously gifted and can basically "turn it on", at will, without even trying.
For most of us mortals, to get a first, we have to work our behinds off.