the only time I've know GCSE's to be very influential in university decisions is for oxford applicants (supposedly cambridge weighs GCSEs a lot less [?]); my rejection feedback after my interview said a part of why i was rejected was down to my 'below average' GCSE scores (all 7-9). outside of that universities seem more concerned with predicted a-levels, personal statement and any admissions tests; e.g. uni of bristol supposedly weighs candidates 40/40/20 on these things, leading to the assumption that GCSEs for bristol arent a point of consideration. may not be the case of all russell groups but if you've gotten into your desired courses and sixth form/college, i wouldnt worry too much