It is not clear whether the OPs question related to "gifted" in the sense in which Betelgeuse takes it - as outstandingly gifted in a way that is going to require special programmes- or as the government definition of G&T, which simply means in the top 10% of that particular school. If it is the second definition, then that will depend very much on the individual school: some schools have a clear programme and a well maintained list, other schools are not very into g&t lists but are very good at differentiating between pupils anyway, or have very good teaching in the top sets, yet other schools do sod all.
But what one could hope for is that a child who us gifted should at least be put on the register and offered some enrichment activities and either be given special homework or that the homework given to the top set should be advanced enough to keep them happy.
It should also be recognised that not all parents who have a gifted child in the first sense find it necessary to have the child diagnosed by an outsider: if they have the necessary background themselves they may feel they know what they need to know and that they know best how to deal with it. And depending on the gift, it may not be possible to cater for it just by extended homework: to seriously cater for a musical gift for instance takes serious money.
I think my parents had good reason to believe I was gifted when it came to speech and languages, but they never felt homework had any particular relevance to that: as academics, they could do far more for me than any school could, so homework was something I got through as quickly as possible so I could then spend time on my own learning.
And for my musically talented brother, they just paid for a maestro. Tbh if you have real musical talent, it is unlikely that any homework you are set is going to make any difference, and to learn from a real master is going to be so prohibitively expensive that no school can be expected to fund that. Dd has a friend who is seriously talented in diving and gymnastics: again, the kind of coaching she needs is going to go far beyond what the school can reasonably provide.
But of course there are many situations where the school can help and should help and a diagnosis may be just the right thing to put pressure on them.