Why fast battleships fit roles battlecruisers could not

Britain could not replace its battle line with many more battlecruisers without losing protection in roles where speed offered little help. A battlecruiser escorting a convoy is tied to the convoy's speed and position, so it cannot use speed to avoid a stronger enemy. Confined waters such as the North Sea or Mediterranean create a similar problem by limiting room to control an engagement and increasing the chance of a close slugging match. Those conditions reward battleship protection, while a fast battleship combines the needed armor with enough speed for a fleet wing or reaction force. A Queen Elizabeth-derived battlecruiser would probably trade a turret for much more machinery, producing a six-gun ship resembling Renown.