British and Union armies across the American Civil War

How would an equal-sized British Army fare against the Union Army during the American Civil War?

Britain would probably hold the early-war advantage through professional experience, but the battle-hardened Union Army could be superior by 1865.

The comparison assumes British and Union forces of equal size rather than the full strategic resources of either state. Early in the American Civil War, the British Army has the advantage because it possessed more professional and overseas experience, including recent campaigning in India. The Union Army was expanding rapidly and had not yet accumulated comparable battlefield experience. By the end of the war, however, sustained campaigning and organisational learning could make the veteran Union force the stronger of the two.