Well, yeah, but 300V allows smaller wires still, and 1000V even smaller, and so on. Hell, you could just wire distribution voltage straight to the socket, make the wires tiny, and eliminate the need for a zillion transformers! Of course, we don't do that for safety reasons, so the choice of socket voltage represents a compromise between safety and efficiency/power. The US and Britain made different tradeoffs, which probably means the constraints were slightly different. I could see "brews tea faster" as a significant factor in a country that once spent 10% of its GDP on tea. That would be a fun fact, if true. But maybe it isn't, and the whole thing is dumb luck and poor coordination around transformer tapping. Shrug.