You need to decide on what kind of Arthur you want. It sounds like the Mallory Arthur, or maybe the Sword in the Stone one? GURPS Camalot will be of assistance with this, and the
Pendragon RPG has a load of detail on high-medieval, glorious and virtuous Arthur.
Then you need to decide on the circumstances that prompt his return. There's a problem, because the period when Britain was most at risk (Summer 1940) is before there were large numbers of Americans involved with the war. However, there is a solution in the
Eagle Squadrons, fighter squadrons in the Royal Air Force composed of US volunteers. The individual heroism of fighter pilots is a lot more like that of a knight-errant than most WWII combat roles.
So if you take that, and an early-morning mission over Somerset, where Glastonbury (traditionally identified with Avalon) is, you can have weird weather forcing landings ... somewhere else. A variant on this, with weirdness on the enemy side as well, would be a German bombing raid trying to destroy Glastonbury, to prevent Arthur returning.
Now, are you going to skate over the issues of language and culture? No plausible Arthur would have spoken anything comprehensible to modern English speakers, and culture has changed as much as language. If whatever has preserved Arthur and his people provides them with modern language and some insight into modern warfare, things will be a lot simpler. It depends how long you want your campaign to run.
Then there's the question of what Arthur does and how he does it. If it is obvious to everyone who sees or hears him that he really is Arthur, then he will be able to exercise a great deal of power very rapidly. But if he provides superhuman leadership and generalship, unopposed by anyone of similar capabilities, then the Battle of Britain will be won quite fast, and then WWII continues on its historical course as a war decided by economic strength.
So that doesn't seem like a very interesting campaign in the long term.
To make it more interesting, have a less superhuman Arthur, and recall some important details of his legend. While a capable warrior, his real strength was as a politician. One of the less obvious points of the stories is that most of them aren't about
him.They are about his knights righting wrongs in a Britain that is basically peaceful and orderly, but subject to local disruption. he true goal of all national leadership - peace and prosperity - is already accomplished.
So a returned Arthur will be trying to arrange matters on an international scale to bring about a stable peace. What he might do is unclear, but it's likely to be more effective than the creation of the modern United Nations by Churchill and FDR. Defeating the Axis is clearly required, but the details of that are likely to be delegated to Lancelot, Gawain, Kay, and so on, with the more junior knights acting as tactical leaders. In tanks, in planes, in ships, and on foot. That seems to create a good supply of scenarios.
You might want to leave out Merlin. In a world of science, he might do almost anything.