A27M Cromwell Mark VI. The Tank, Cruiser, Mk VIII, Cromwell (A27M), named after the English Civil War leader Oliver Cromwell, was one of the most successful series of cruiser tanks fielded by Britain in World War II. It was the first tank in the British arsenal to combine a dual-purpose gun, high speed, and reasonable armour. Late in the war it was the most common British design in use, replacing the Sherman tank for some units. Its design formed the basis of the formidable Comet.