If your DC loves maths and you live in Cornwall, Devon, Dorset or Somerset, I can heartily recommend the Exeter Maths School State run, it offers a weekly boarding option to those who do live further afield.
It's an amazing place where the students can be themselves and rules are only there when strictly necessary. It's an amazing immersion in maths in all its glory. The staff are wonderful and go above and beyond all the time. The results at A level are fantastic and they get one of the top value added scores in the country.
All students do A levels in Maths and FM and either Physics or Computer Science or both. In addition, a 4th A level can be taken at Exeter College. All students do the Exeter Maths Certificate which is research on a project of their choice and helps to prepare them for Uni.
I don't work there. I'm an ex-parent. I cannot fault them in any way. It's a wonderful school and I'm so grateful that my DS had that opportunity.
Should also mention that for younger ages and for South West maths teachers, they also have outreach courses.