My DD was on her primary school register as AG&T, but other than letting her go on the state run courses run at the AG&T centre in our county (which we paid for), the school offered nothing additional at all.
The Special Needs teacher spent the vast majority of her time working with the under achievers. They even had DD helping the less able children with their maths in a lunchtime club. Felt a bit like a Victorian village school situation with the older children teaching the younger.
It depends a lot on the individual teachers in the school, if they are prepared to do the extra work involved.
We moved her for Y6 due to bullying and the second school was fantastic, giving extended work. Luckily there were several other children in her class who were also very bright. They worked in a little group on the extended work and really enjoyed it.
Find out if your county offers AG&T courses. They are normally one day, but can be more. Some were at weekends, and some were in term time. DD loved going on them.
You will probably have to do extra stuff at home with your DC to stretch them. Join your local library if you haven't already. Take them there to choose their own books which will probably be ahead of what the school is giving them. Once they are reading more confidently, you will find many books are in series'. Order them from your library online and then collect them when they arrive. Then you access the books at all your county's libraries. I used to collect around 20 books at a time for DD. She used her own ticket and my DH's ticket too. Not as much time for reading now she is at senior school.
For maths, get the school to subscribe them to something like mymaths online. My daughter loved learning online. There are also other maths sites for children which the SN teacher can advise on.