Which country was leading "rules-based order" of the west before the USA?
Which country was the leader of the West before WW2 and before WW1?
The kind of cooperation that exists today between the western countries is a relatively recent phenomenon, but its underpinnings can be found in the 1814 Congress of Vienna and Treaty of Paris in which the European powers came together after the Napoleanic Wars to discuss the prevention of future wars of that scale and proposed that a "general congress" be held between the warring powers. Prior to this time, while there was a general principle of non-interference established by the Peace of Westphalia, this marked greater integration and consensus on international norms as far as the "rules-based order" goes.
At this time (early 1800s) the UK was the world hegemon, and would remain so until the US began to take this role with the world wars. As the dominant European power it was the defender of this western "rules-based order" until the US took the mantle.