Are you really asking a different question? Are you really asking who has a better army in 1815, rather then if they fought a war at that time? Having a hypothetical battle is a different question then fighting a real war, in a real place. The outcome of a real war depends on the circumstances of the conflict. How does the British Army get to Poland? Why would they even want to get there? The British still have many of their best units fighting in North America. In a war between these countries over Poland, I would imagine no British Army would be sent. The British would send money, and the RN would raid in the Baltic, and Black Sea. Prussia would side with Russia, and tip the balance of land forces against Austria, in Poland. Blucher would fight Wellington in Belgium. France under Louis XVII would side with Austria, invade the Prussian Rhineland, and send a Corps to support Wellington in Belgium. The big winner would be Napoleon.