Hey, I just finished a maths degree and am about to start a pgce in a few weeks.
There are different routes into teaching secondary maths.
The first and most common route is to a do a degree in maths (with at least 50% being maths and the rest being another subject or maths). Then doing a Secondary Maths PGCE. This usually takes 4 years. (just to note that a PGCE can be worth up to a third of a masters now, so if you want to do a masters later then this means you can use the pgce as some of the credits).
The second route is to do a degree, then you can do an enhancement course for six months, then a Secondary Maths PGCE. This means you can choose whatever degree you like, then do a sort of maths top up in between the degree and PGCE. This usually takes 4.5-5 years.
The third route is to do an undergraduate degree in secondary education, specialising in maths. These are usually four year courses and give you the QTS (qualified teacher status) as part of the qualification, so you don't have to do a PGCE, but you can sometimes do them in 3. They do however give you only a BEd (so nothing towards a masters like the pgce).
The forth route is to find employment based training. Usually a degree in maths is required first, and then you can do the training in a school (much like a pgce but as a job and without the academic side, so more vocational) and get qts in about a year usually. These are harder to come by and are more like jobs so places are limited. Sometimes employment based training is available for those without a degree or a non-maths degree but these are less common to come by.
If you are not certain about teaching but are certain about maths then I would def recommend the first route as you can always use your maths degree in something else, whereas finding a job in the maths field with an education degree is far harder. If you are unsure about maths and teaching then consider the second route, as it is only an extra 6 months (and you still get grants, etc, to do it).
Sorry that was really long, hope it helped.
Just so you know, you only need above a C in GCSE maths and english technically (and a-levels don't matter) to be a maths teacher, but in order to do a maths degree you would need a-level maths. And in order to qualify for teacher status you would need to complete some more advanced level maths and teaching experience. So in a round about way you do need maths a-level and a degree but it isn't absolutely necessary and there are ways around it if need be.