Mythology loving Britfag here, seeking to do my great nation justice within this Awesome idea.
One of the oldest Myths is that the men of Albion (Britain) were descended from giants, who in turn descended from a group of exiled Atlantian princesses and Incubi (No, I am not making this shit up). Perhaps, to aid the war effort, these great ancestors are awoken, the stone circles which mark their resting places slowly heaving asunder to reveal the titanic forefathers of Britain.
On the other hand, a great flood sweeps through Britain, focusing on and nearly drowning Glastonbury. No one knows what's going on, and the government expects a Nazi occult attack.
Then, King Arthur and all his knights ride out, a huge procession of armoured riders on horseback. As they ride around the four nations of Britain, more and more heroes join them, from the scoundrel Robin Hood to the latest trueborn hero, Jack Churchill, and then turn their crusade to the mainland, for the last great war.
Meanwhile, Ireland blooms, the ancient fae emerging from hiding to protect their beloved land. Of all the world, only Japan is safer from occult attacks, for after the many magic wars they fought of old, the Irish fae are stern and resolute soldiers. However, even the Fae knew the war needed all the help they could give, and so passed to the British army both their contingent of reborn heroes, and the four treasures of Ireland- The Dagda's Cauldron, the Spear of Lugh, the Stone of Fal And the Sword of Light of Nuada (Possible plot hook: Guarding one of these powerful magical items during transport)