The Order of Excalibur - The Ring of Fate

The Order of Excalibur - The Ring of Fate is a fantasy novel for young adults set in Britain early in World War II. At the height of the Blitz and the Battle of Britain, young Canadian wizards Nick and Ginny are recruited by the mysterious Order of Excalibur as the country faces the threat of invasion. Nick and Ginny's father, Robert Jennings, is the world's leading scholar on the magic and folklore of the Arthurian period, based at Cambridge University, while The Order of Excalibur has kept the secrets of Merlin since the early Middle Ages.
The Order also has possession of a ring that Merlin supposedly used to materialize a stone circle similar to Stonehenge known as the Ring of Fate in the centre of his workshop. The spaces between the stones in the circle would reveal how events might occur if certain decisions were made. However, Merlin's ring hasn't been used since he died, and the stone circle hasn't been seen since that time. The Order made attempts to materialize the Ring of Fate when Britain was under threat from the Spanish Armada in 1588 and from Napoleon in 1805 without success and when the situation in 1940 is even more desperate the British government is willing to try anything.
Robert and the children begin working in an underground facility in the heart of London during the Blitz and the Battle of Britain. Using their magical powers, Nick and Ginny are able to get the Ring of Fate to appear. While they view potential scenarios showing that all will be well and that there won't be a German invasion, they also see other possible outcomes revealing a successful German invasion and subsequent occupation of Britain.
When a massive explosion almost destroys the underground facility, Robert, Nick, and Ginny are shocked when reaching the surface that London is now under German occupation. Robert, Nick, and Ginny attempt to escape from the German soldiers but soon begin to forget the other timeline and their own identities, as they go on the run in a nightmare world.