How would the British Army fare against Civil War armies?

How would the British Army have performed against the Union or Confederacy in a land campaign?

Britain likely would have done well against either side early in the war, but the mature Union Army of 1865 would probably have defeated it; the weakened Confederacy is less certain.

The comparison changes over the course of the war because neither American army remained static. At the beginning, the British Army's recent combat experience and professional organization would likely have given it an advantage over both improvised American forces. Four years of mass warfare transformed the Union Army into a large, integrated, battle-tested organization with infantry, cavalry, artillery, and mature command systems. That mature force resembled the New Model Army at the end of the Wars of the Three Kingdoms, whose combined experience made it exceptionally formidable. By 1865, the Union Army was probably the world's best, while the depleted Confederacy remained a less certain comparison.