Why Lion was not simply a British Iowa

Was the Lion class the British equivalent of the Iowa class?

Not directly: a successor to Lion would be closer, and British fast-battleship doctrine differed enough from American doctrine to make simple equivalence misleading.

Vanguard is identified as the immediate British fast-battleship design. A successor to Lion would be a closer British counterpart to Iowa in the fast, heavy battleship role. British and American doctrine gave their ships different priorities. Small differences in emphasis could produce larger differences in design.