It’s an old saying. The Empire had colonies all over the world so at any point in time it was day for at least one colony. Therefore the sun never set on The Empire as a whole.
The joke is making fun of that old saying, pretending it means there is no night for the British isles and all the sun is causing the temperature of the isles to increase.
I hope that’s enough explanation to remove any humor from the joke.
They’re all fried tubers. I’ll let it slide. The real travesty is no fried yuca on the list.