Was the Royal Navy's Devastation class a coastal-defence design?

Would the Royal Navy's Devastation-class ships count as coastal-defence ships?

No; they were experimental and imperfect, but they were designed as battleships rather than for a distinct coastal-defence role.

Classification again depends on intended role rather than simply on size, range, or poor sea performance. The Devastation class experimented with important design ideas and was conceived as a battleship type. The result was an imperfect battleship. That shortcoming does not retrospectively turn it into a coastal-defence ship.