The H3 battlecruiser answers a need for 18-inch gun power

What scenario would make Britain choose an H3-series battlecruiser design?

The Royal Navy would need to believe that heavily protected opponents required 18-inch 45-calibre guns, making the H3's concentrated armament and protected internal arrangement worthwhile.

The completed 18-inch 40-calibre gun does not match the longer 42- or 45-calibre weapons considered elsewhere. The H3 makes sense if the Navy expects to defeat a heavily armed and protected opponent that justifies the larger gun. The all-forward version also separates machinery from magazines. Angled armour and the compact protected zone make the arrangement more than a simple exercise in mounting the biggest possible weapon.