The second life of old naval guns

Could 12-pounder naval guns have been useful as early Second World War anti-tank weapons?

Potentially; six-pounders did receive a second life in that role, and a 12-pounder would have been a credible weapon to consider.

Six-pounders received a second life as anti-tank weapons, and a 12-pounder might also have had value. A naval gun may remain militarily useful even when its original ship no longer does. Calibre alone is insufficient, because ammunition, mountings, transport, and a credible land or coastal role must also survive. A reserve of older guns works best when those supporting requirements are planned before the emergency.