Every year hundreds of good applicants are rejected because they either didn't read the GCSE requirements - or they thought that somehow this did not apply to them.
GCSEs matter. If the Uni says it wants 6 A grades and/or A grades in specific subjects and you dont have them, your application will go straight on the NO pile. it doesnt matter how good your A level predictions are. if you fail the GCSE test, your application will stop right there, without the Uni looking any further or even bothering to read your PS.
This means you can fail to get an interview just because you have GCSE English Language at B and not A. Yes, it really IS that basic. So read the full admissions requirement properly, and never ever assume that they wont notice that B grade. They will.