Why not rebuild with heavily armed monitors?

Could Britain replace its losses with heavily armored monitors carrying eight 15-inch guns?

Not as the main solution; a ship with that armament is effectively a battleship, and Britain has the infrastructure to build full ocean-going dreadnoughts.

The suggested shallow-draft ship combines heavy armor with eight 15-inch guns, making it a battleship in all but name. Designers might explore heavily bulged hulls or compact triple-turret arrangements, but Britain still needs ships that can perform the full battleship mission. Its industrial base can build multiple dreadnoughts at once, which makes a specialized substitute less attractive. Emergency expansion might even bring facilities such as Thames Ironworks back into capital-ship construction.