Most Unis don't actually look at AS grades - because not everyone takes them, and therefore using them as part of formal scoring wouldn't be fair or equitable. And for GCSEs, they usually only assess your top 5 or whatever grades, and this forms very small part of the overall weightings, so one random D grade is not going to be a deal breaker. As an example, Bristol uses these weightings for Law - 20% GCSEs / 40% A levels / 40% LNAT.