Why HMS Unicorn was not classified as a battleship

Was HMS Unicorn nearly classified as a battleship because spare battleship tonnage was available?

No; the battleship-tonnage story is a myth, and Henderson obtained Unicorn as a forward aviation support ship.

The story that HMS Unicorn was almost classified as a battleship because spare battleship tonnage remained available is rejected as a myth. The battleship category did not contain enough tonnage for her. Henderson instead secured the ship as a forward aviation support vessel, a designation that matched her workshops and stores while leaving a usable flight deck. Treaty limits and administrative classification therefore shaped the route by which this flexible aviation ship was approved.