Wow. That's rare. A Switch game reaching number 1. Well it is a quite time of year for big releases.
Sushi Striker The Way of Sushido was a complete flop. It's not even in the Switch specific chart.
Correct me if I'm wrong, but I believe this is the first Switch game to hit number 1 in the UK that isn't an enhanced port of a last gen game (like Mario Kart 8 Deluxe was)