Chartered Accountancy firms put a lot of weight on A-level grades as these have been found to be the best predictors of success in professional examinations.
If your A levels were not outstanding, this may sometimes depend more on the quality of the school you attended than your ability. In some inner-city schools it may be that a mediocre A-level performance that you achieved might have been the best in your school - if this is so, then make it clear. An average performance at a weak school academically may be the equivalent of a much better performance at some prestigious establishments.
Similarly, if there were any other external factors, such as illness, that may have affected your grades, tell the interviewers - but don't sound as though you are making excuses.