In short, if you can improve something on your CV, you should. Not that it matters too much in practice - but you don't get a second chance to make a good first impression. Someone with low A-Levels or with a Class III/II-2 University degree gives off the impression of not having been too committed to whatever he's studied. Which leads to some awkward conclusions. Did he just randomly pick a subject which he didn't enjoy and was therefore not motivated to do well? Is he one of those people who are happy as long as it's a pass, even if it's just a marginal pass? Sure, some recruiters won't care, but the more meticulous ones might get the wrong message out of it.