Upthread my observation that applicants with strong EC seemed to do better in gaining places on some seriously competitive STEM courses than other equally academically qualified applicants , did not suggest causation.
DD has gained a lot from EC, which she partly took up at a point where she was struggling with dyslexia, wanting to have something she was good at. Perseverence, juggling (heading to the library at lunch time to do her homework), access to a wider peer group and to adults who were not teachers, coping with group dynamics in a competitive environment, learning to listen and follow instructions, experience of travelling in a group and staying in hotels...that sort of thing. Also deciding what she wanted to do and sticking with it.
Will this have helped at interview? Yes, because she is used to talking confidently to a range of adults. Will this make her a better student? I think so, as she has good experience of getting on with things, coping with disappointment and adversity etc. We have come across a few kids for whom grades have been everything, sometimes by working very very hard to the exclusion of almost everything else, sometimes propped up with out of school support like tutoring, and have wondered what will happen when they stop being top of the class.
That said the same skills can be learnt in different ways. Being a prefect, volunteering, responsibility within a family, engagement in the community.
KCL medicine puts it quite well:
We look for applicants who have participated as fully as possible in school or college life, making the most of the opportunities available to them and also demonstrated some experience of society beyond their immediate environment.
Haybott will know better, but I can see advantage of a brief mention of non academic experiences/achievements and the skills gained which would help you thrive at the next stage.