DD is top 1% and was in a decent state primary: I would say her experience was positive overall, though she was sometimes bored, and rarely working at full capacity. Despite that, there is a lot of the main subjects (esp maths etc.) that they can’t easily learn at home, and there is usually more than enough in the primary curriculum to occupy a very bright child. There are lots of group skills they learn together, and usually something like music, a play, clubs etc. going on to add extra to the core school curriculum.
Teachers should normally have greater depth and extension tasks available for each lesson (though annoyingly the national curriculum changed a few years ago from requiring differentiation beyond the child’s current level, to only requiring differentiation within that level, eg. greater depth within the topic rather than extra content). However, DD’s school did often try to add extra content for a few children. She was lucky in that there were a number of other bright children there too, who whilst not quite always working at her level were close enough to provide a bit of group stretch and competition (especially in maths and science).
It often also depends on the individual teacher: DD would typically find that every year she would get put back on some reading level she was way above, and it would take a while before the teachers properly assessed where she was at.
School isn’t all about academic subjects, though. The social and emotional value of learning how to learn in school, plus how to navigate friendships, rules, and the social environment is invaluable, and even extremely bright kids can struggle a lot with those things and need to learn them along with their peer group.
If the child is very bright, there might be a certain degree of having to put up with being a bit frustrated in order not to miss out on the social and emotional benefits of being with their peer group — something that is a trade off that parents often don’t quite appreciate. I’ve met many children who have been accelerated academically, and they have a whole load of other, different frustrations and problems, some of them less easy to solve. My own opinion (also as a former gifted child myself!) is that sometimes being a bit intellectually frustrated in primary school, because you are too clever, is actually not that bad a problem to have as problems go. Primary school is also a place to feel comfortable, try things, have fun, have friends and belong to a school community, not just do school work.
DD got a scholarship to an academic private secondary, and I would say that their junior school doesn’t actually provide anything particularly beyond the level of the state primary curriculum — EXCEPT for languages, art and music, where there is normally much more provision than in a state primary. If you need to give your child extra stretch at home, language learning is a good way to go: a French conversation club for example, or an instrument. Your DD might find starting to learn piano or another instrument now may give her some of the extra stimulation she needs.