Need advice from top unis in uk

I’m aiming to get into a top UK university for mathematics (Oxford, Cambridge, Warwick, etc.) with the goal of eventually breaking into quant finance, and I’d love to hear from current students at these universities. How much do my 5th year summer exams actually matter?
I find the Leaving Cert fairly straightforward and I’m confident I could get 6 H1s with around 2 hours of study a day. Is it worth putting in that much effort for my summer exams. Additionally what else could i do to maximise my chances at getting into these unis. Ik i have to do entrance exams but from what ive seen theyre not as difficult as people make it out to be.