Lurked,
i think it is difficult because of the slipperiness of the term 'gifted' or 'most able'.
The old G+T 'top 10% of any cohort' thing - 3 in every primary class - is clearly bonkers, both because a particular cohort may actually have 0 gifted children, or 5, and because the vast majority of the top 10% actually don't need specifically different provision.
Some authors use a different scale of giftedness - 1 in 100 (a couple per year in all secondaries, 1 per 3 years in a primary) 1 in 1000 (1 or two per school for secondaries, 1 per career for a primary teacher teaching classes of 30), 1 in 10,000 (one per 10 years in many secondaries, most primaries will never see one). That can be more useful, because it helps us to see how 'different' such a level of giftedness is, and whether there is any point in designing any mass education system around their needs.
Very high levels of giftedness are probably best thought of as a 'special educational need' - some superselective grammars serving very wide areas are possibly the closest we get to a 'special school' serving the needs of this group, though their identification procedures at 11+ are woefully imprecise and probably only xcapable of sifting out those 'of 1 in 10 giftedness who perform well on the day' rather than true 1 in 100, or the 1 in 1,000 pupils who might really NEED such an education.
Otherwise, we are down to individual provision. Those who I know who are in those very high levels of giftedness tend to be accelerated - either generally, or in their specific area of giftedness (they can be average, or even below average, in other curriculum areas).
So I think it is reasonable to complain if a school does not provide routinely well for those in the top 10% (1 in 10); reasonable to expect secondaries to make adequate provision (probably informal, rather than through adapting the formal qualification routes) for the 1 in 100 level, but for the 1 in 1000 and above, invidividual, tailored provision for this level of acute special educational need is needed and should not necessarily be expected from mainstream schools.