This is not the US, I don't think anyone really reads the personal statement carefully. They only scan it, basically for any grammatical/spelling errors. A half decent one will honestly do. For me the Maths challenge was just a one sentence in my whole statement. I just made sure my passion for maths shone through.
For the top universities, the poster above is correct - focus on your grades and MAT/TMUA and areas around the A levels. Avoid the London Unis as they want mostly international students . Personally I think the Cambridge route is unnecessarily cruel (Oxford is easier and fairer) - I am glad I failed the Cambridge interview as I don't think I would have had the capacity to study STEP on top of the 4 A levels. I had a classmate who passed the Cambridge interview and focused so much on STEP, he didn't get the further maths grade he needed for his insurance (Warwick) and it didn't even make a difference as he didn't pass the STEP exam anyway - he had to go through clearing and went to Lancaster in the end. He went into clearing with 2 A*and 2 A!He is still feeling upset about it and keeps talking about doing his masters in Cambridge! He even appealed to Warwick saying that if the grade boundaries were the same as the prior year he would have had an A in further maths, but they said no. It was the same for me, if the same grade boundaries applied in 2023 as in 2022, I would have had 4A* but I did get into Bristol as my insurance choice and I am very very happy! With that in mind, make sure your insurance choice is a proper insurance, with actual lower grade requirements.