In the very top universities, where all applicants will be expecting AAA or more, and where it's grade not point-based, I think some (not loads of) extra curricular activities can help to differentiate between otherwise identical students. However I think anything showing a real interest in the chosen subject is most useful, such as attending summer schools, exhibitions, work experience.
Best extra curricular activities to list, aside from course-specific, are those that demonstrate commitment, team work, leadership etc. Also useful when it comes to writing CVs and getting jobs in 4 years time.
Once you look at less competitive (but still RG) universities, I'm not sure extra-curricular is quite so important. Once you go below RG then they are actively looking for people, so if you have the grades/points then you are in.
Been looking into this a lot recently as I volunteer mentor students through UCAS. I went to 2 highly competitive RB universities and my peers and I were almost without exception AAA + DoE, continuing a sport at a reasonable level + frequently music. Didn't need to be at top level for hobbies, though some people were county players etc. The range of activities is probably also a reflection of the kind of backgrounds most of my peers came from, rather than a marker of superiority.