Should i defer my uni entry?

basically I want to be a maths teacher and obviously I cant do work experience in summer holidays? ive helped a year 8 class at my school when I was in y12 and going in to y13 I'm going to be helping some other classes. I also do some tutoring outside of school (not much).
I'm applying this time to uni to study mathematics and then plan to do a teaching course afterwards. thus, do I defer to do a year work experience before I go and study maths? then can go straight to the teaching? or do I do a year experience between the maths and teaching courses?
I just want to get a lot of experience in different locations and obviously its hard to find experience in summer holidays for teaching?

You should be able to get experience during term time alongside your degree. Once you have your timetable each semester approach local schools to find out how you can help and what they need in the days you're available.

thus, do I defer to do a year work experience before I go and study maths? then can go straight to the teaching?

You still need to do the PGCE; you can't just go straight into teaching.

You get experience along the way, e.g. helping out at schools local to your university during term time, summer opportunities like TEFL and Camp America, placements on the PGCE, etc. Maths teachers are in high demand so I wouldn't worry too much about having lots of experience - just get your degree first.