If you want further reading etc, I remember there was a series of lectures recorded by an Oxford maths lecturer targeted at a-level students - I watched them maybe 2 years ago??
also STEP foundation courses are a great way to push yourself a little bit for fun - its the Cambridge entrance exam course, but its just a bit of fun if youre a maths nerd!
I also participated in the Cambridge Women In Maths programme, which I think happened early in year 13, but applications were definitely in year 12!
Also AMSP may run courses in your area, in person or online, that you might be able to sign up for - very interactive, team work, problem solving skills using STEP and MAT past papers. nrich also have some problems under post-16 on their website!
A lot of these definitely seem very oxbridge centred, but it was my experience that they were an interesting way to push yourself, and were great to put on a personal statement, and you dont have to take them too seriously at all if you dont want to go to oxbridge! honestly the AMSP classes in particular were just fun and chatty whilst also getting through some maths
also kind of like revision without the boring repetitiveness of textbook questions!!!