To answer your questions the past few years the cut off score for GCSE grades at Cardiff has been 26/27, this is where an A* is 3 points, an A is 2 points and a B is 1 point, they score your best 9 GCSE grades to include maths, english language and the sciences, but it is important that they dont count clashing subjects such as maths and further maths.
Some candidates each year get interviews with lower than 8A* 1A at GCSE due to really great personal statements.