As ever, you can get a job whatever your academic qualifications. For "graduate" positions, by their very nature, academic qualifications are an important part of the criteria, as they are aimed at graduates. But in the end, for many jobs the academic qualification itself doesn't define how good the person will be, and so the real test comes in the interviews/assessment centres, as which point the academic achievements are almost irrelevant (except in the sense that they may give a sense of lack of drive/ability to focus on a task etc). And being good academically doesn't always help. Had a guy in an assessment centre yesterday who was clearly very focussed academically, but he struggled to demonstrate an awareness of the business world or to express what non-academic skills he might be able to bring to the role (which was unrelated to his degree). He was also a bit overpowering towards others in group situations, so despite the fact that I'm sure he's doing excellently in a tough degree subject at a decent university, he wasn't what we were looking for.
I can't quite remember where this whole UCAS point discussion thing started, but here's my view on it, from a business angle. If you are still in a position to earn UCAS points, get the best you can, as it will open more doors for you later in life. If you already have your UCAS points, and you don't have a great score, then there will still be good opportunities available for you, so go for those instead. But once you are past the application form stage for most of these jobs, remember that it's not about exam grades any more. You need to show that you can adapt your life experience (which, yes, may involve challenges that you've faced in your academic work, as long as you can show what business skills you displayed) to the role that you are applying for, and that you have decent social/group interaction/teamwork skills.
And of course further down the line, once you have a couple of years of experience behind you, UCAS/degree results become less important than the fact that you can demonstrate strong performance in a business scenario, so the company you are applying to doesn't have to gamble on how you might adapt any more...
As for why the Big Four might be relevant, take a look here:
http://www.top100graduateemployers.com/employers/
You may not want to work for any of them, but considering the top three positions are taken by members of the Big Four professional services firms (and Accenture at number 4 grew out of one of the Big Five (when they were that), it is notable. But, of course, not the be-all-and-end-all.