In a festival often dominated by heavyweight pop, Tanner Adell’s mid-afternoon set on the Rainbow Stage offered a thrilling detour into rhinestone-studded country pop — with a Gen Z twist and plenty of attitude. On a stacked BST Hyde Park Sunday that saw Sabrina Carpenter headline the Great Oak Stage for the second time of the weekend, it was Adell who delivered one of the day’s most memorable performances before the heavens opened and drenched London in a biblical downpour.