Agree with @hebegebe that math competitions are better indicators of high ability. I also reside in the UK and my D22 went through the UK system so I am familiar with the UKMT competitions. Your son’s score should have qualified him for the Olympiad round - how he did in the subsequent round will provide a finer indication of his talent (and, of course, he can skip ahead to the Senior Challenge if he’s that far ahead of his peers).