The best success with this was kids we had at private schools (we used both private/public depending on the child and what they needed/wanted).
While the schools stretched in maths by dividing into cohorts with one being advanced, I can’t recall any extension or differentiation with English. However, I had one maths whizz who quickly went through the work easily, but could not ride a bike, or indeed tie their shoelaces (not for our lack of trying, so they were given dispensation to wear those elastic laces that didn’t need tying). So, every few maths lessons the pastoral care team would organise personal bike riding lessons, did intensive work with shoelaces etc. This benefited them way more than getting more maths work and being well over a year ahead of the average cohort. Extension does not always need to relate to maths/english and education can be viewed in broader terms.
Another, at another private school, were in a group identified as being ahead academically, and again, while given some higher level maths stuff, instead of pushing them years ahead further than the average of the cohort, they did another language again with them to extend them in another way.
Don’t have any experience with this in our public system, as those who went through public were average academically so it was not really relevant. To be clear, that’s not why they were public, it was their choice, they wanted to follow their friends from primary and as they had no specific pastoral care needs we were happy to let them. Although, in our system (both public and private) maths has to be differentiated after the second year in high school so I guess that would assist those who had waited until then?
ETA- I now recall in our public, the schools we used did some composite classes, which one of ours was always in. Guessing this may have been an easy way of extending some kids by mixing them with the grade ahead and that work but I can’t recall the details as was quite a while back.