Hi, your daughter sounds very like mine, but mine is 'up a year', so in Year 2. As we're overseas, I'm not au fait with the NC terminology or mileposts, but she's a fluent reader, with an excellent vocabulary and comprehension skills, her teacher says she probably has a reading age of 9 or 10, whatever that means. She and another girl in the class also get 'extension' language homework - so more complicated spellings / tasks to do at home each week, so I certainly know she's not struggling academically. That said, although she's in the top quarter of the class for maths, and the teacher describes her as an 'all-rounder' she's not getting additional extension work there.
I have spent hours agonising over whether being the youngest in a class is good for a child, especially as most educators say it's not. However, my daughter is an exceptionally aimiable and gregarious little girl, so the social side of school has never been a problem. The teacher she had last year wasn't as good and she would just tell me that my daughter was going to 'waste her potential as she was bright but was too busy chatting to work'. This year's teacher knows exactly how to keep her interested and is getting far more out of her as a result. She also says she shouldn't move down a year - yet.
The 'yet' comes as I do believe, as does DD's teacher, that in the long term it's better not to be one of the younger children. Not necessarily in academic terms, but the youngest is inevitably going to be smaller, slower, less coordinated, more easily tired etc etc etc. With every month that passes there's less of a difference between the boys at the bottom of the class and the girls at the top. Everyone'll be able to read and write well by the time they're 7. So my plan is to slot her back into the 'correct' year when we move in the next year or so. By then, it's not like she'll be reading chapter books while everybody else is saying 'a for apple' as would have been the case previously. By Year 3, I'm hoping that it'll be easier for the teachers to tailor work more specifically to each child's ability. And in our particular case, it means life at school should be a breeze while we settle back into the UK after 4 years away.
If I were you, I wouldn't push to move your daughter up at this stage, I'd ask what additional support the current teacher can give to challenge her within her own class. I'd also ask these lovely ladies what other resources there are out there for you to use with her at home. (and I'll be checking back for next year!)