Sorry but are we equating the normal senior maths challenge to an Olympiad here? They are 2 very different things. Only the top 1% of scorers in the senior maths challenge qualify and participate in the British maths Olympiad. Having something like that certainly would be attractive for the top unis but you definitely do not need it (not a lot of people have it anyways due to how difficult it is to get in). A normal gold in the senior maths challenge is a nice thing to put in your personal statement though and shows you have a strong mathematical ability, it’s also not true that everyone applying would have a gold in the senior maths challenge.