BA system is reasonably fair with C's or a 2:2
A levels are a passport into University. Those with better A levels get into the better Universities which normally will offer harder courses etc.
What they are doing is saying ok you may have had a disaster at Uni but u do have the 2 C's or above, and could have been pressure of the being at a "better Uni" and u obviously can excel through study.
Or saying u have had a disaster at A level, continued into higher education and rectified it by getting a 2:2 or above.
In all honesty I believe that if you meet the criteria one or the other, they will not hold it against you. The interview is much more than just academic grades, I know people that were hardly asked on them. They are much more concerned with how u interview under pressure on a whole variety of subjects topics.
Best of luck
Sagey