Could older British battleships accept the 15-inch/50?

Could the Queen Elizabeth and Revenge classes be refitted with Britain's proposed 15-inch/50 gun?

Possibly, especially for the Queen Elizabeth class, but the longer barrels would require rebalancing and stronger turret machinery, and the work would be expensive.

The longer barrel adds weight and moves mass farther from the mounting's balance point. The roughly ten-foot increase from a 42-caliber to a 50-caliber barrel would change both static balance and the momentum the training and elevation machinery had to control. Queen Elizabeth-class turrets might have accepted the change during a major refit, but their mechanisms would need strengthening and careful recalibration. The Revenge class is less certain, and fleet standardization would demand expensive engineering rather than a simple barrel swap.