Sending part time before compulsory school age is a perfectly valid and legal option. You have a legal right to be this as outlined in the Schools Admissions Code, page 25, section 2.17 C. They legally cannot stop you, though a few link to try.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1001050/School_admissions_code_2021.pdf
You know your child better than anyone.
If you’re concerned about maths speak to the teachers about what they’re covering and do it at home. There should be a curriculum overview on the schools website.
My top tip is to count at home, as much as you can. Count steps on the stairs, count forward, count backwards. The order of numbers is something we must learn by rote. You cannot count too much. There’s not a huge amount of time to practise counting g at school. I used to get my class to count as we walked to PE, etc. A key thing that stood out to me in teacher training was that, especially in maths, you need a really strong foundation to build future learning upon, and in my experience, that foundation is solidly knowing the order of numbers. Yes there are patterns that develop, but you have to learn a certain amount by rote before the predictable patterns emerge. (Ex Primary School teacher here).
It is far easier to teach a child one to one, through play, than it is to teach 30 children at once.
It’s my personal opinion as a teacher and as a parent that we start school to early in this country. Some of the expectations in the first few years of school are not developmentally appropriate for all children. If you have a legal right to part time, grab it with two hands and give your child a gentler start at School.
Trust your instinct. Listen to your child. You got this!