Modern coastal batteries did not depend on medieval castles

Did the loss of old coastal castles deprive Britain of ideal shore fortifications?

No; many survived, and modern batteries could be built at the necessary coastal site whether an old castle stood there or not.

The premise is corrected because many British coastal castles and forts still existed, sometimes in private hands and outside familiar public sites. Britain historically relied heavily on the Royal Navy, supplemented by fixed batteries at important approaches. During the world wars, modern positions could be added beside older works such as Pendennis Castle. The disappearance of a medieval structure therefore would not prevent engineers from siting a new battery on the same useful ground.