RecruitGal may well have inadvertedly hit the nail on the head in mentioning a "screening" method. With hundreds of well-qualified candidates with good degrees to choose from, it's not that surprising that they might choose criteria such as this as one means of cutting down that pile of forms to a more manageable size. After all, all other things being equal, why not use A-Levels as an indicator? If nothing else, it might be considered evidence of consistency. As for whether it will count against you in this case, who knows? 24 points not all that good these days tho and wouldn't get you in to most courses at most top universities TBH.