oddsocks I knew a couple of parents whose children did GSCES last year who were very concerned about their children not hitting predicted grades from the Mocks scores. In all cases their children on the surface denied it mattered and were thoroughly bolshie when questioned/advised/given gentle warnings and talkthroughs about the future outcomes. However they then received excellent gsces, which made the parents feel a bit guilty for laying the pressure on. They certainly didn't appear on the surface to do what the parents thought they should be doing revision wise, but it somehow came out in the wash. I think you can lead to water but you cannot make them drink. They are probably listening but don't want to appear to.
Another friend's daughter became so anxious she did not even take her Gsces in the end through pressure to receive As from the school, who kept trying to suggest she did them in room by herself, various failed pastoral interventions, because they didn't believe quite how anxious she was, being a model student. Yes Gsce results matter, but nothing matters more than your child's mental health. And if there is a good enough Sixth Form, she has a second chance to shine doing A levels, even if she/he doesn't go to selective sixth form (which is probably only getting better results because they select)
Dd has mixed predictions and pretty mediocre mocks. She has a conditional place at two places which need higher grades, she is v keen on one. I am now leaving the ball in her court. If she wants help I give it, but otherwise I can do nothing, she is going to do that work if it needs doing. Ds2 desperately needs a 6 in English to do his A level choices, but if he doesn't take my advice about how to get one, well that is his choice and probably will mean he is in a different Sixth Form college, which is also the right place for him.