GCSEs are too easy, you can get straight Cs at least without revising. Bs with a little revision, and decent As with a few weeks of decent revision. Tbh, they never really matter unless you're looking at Oxford/Cambridge.
Well, a D in any of the core subjects will stop you from becoming a primary school teacher should you choose that as a career path post-university.
I remember for one of my English or Science papers - can't remember now - I'd completely missed out the last question on the back page. I only realised it was there when I handed the paper in and saw it staring me in the face. So, that's a great piece of advice: read the paper carefully and leave yourself time to review it at the end!