Crank from what I can remember
Airlines generally prefer little flying experience, ie you have had a go, not picked up too many bad habits. It is rare for those with CPL, half way through ATPL exams to be accepted (don't know why TBH, I might be totally wrong there)
BA 2 C's at A level (not including General Studies) and/or a degree with 2:2. You need one or the other, I know people that have been invited to interview with above 2 C's at A level but left Uni early etc.
BMI 2 C's at A level pref in Science subject + Maths. Degree Pref
Air2000 2 C's at A level pref in Science Subject + Maths. Degree Pref
Britannia: 2 out of the following A levels Grade C and above. Maths, Biology, Physics, Economics, Chemistry. Degree in 1 of the 4 subject pref
Flybe: says 2 A level passes pref in Science Subjects, I think it considers C's as passes though.
Military just need A levels
Once you have a license most airlines wouldn't give a damn about A levels, you have proven that you meet the academic standard for flying.
Hope that helps
Sagey