I don’t remember much about the novel, but I think it may have been for young adults. I read it in 1991, and is likely a Scholastic book. The main characters find clues and determine that there’s going to be some kind of attack on London by the Germans. Towards the end of the book, they steal a fighter plane, and take off in the early morning to try and stave off the attack. They’re expecting to see German aircraft approaching, but only see a lone British Lancaster bomber heading to London. They head towards it, make contact with the pilot using hand signals, because the bomber signals that their communications equipment is damaged. The main characters can’t figure out why there are no German aircraft heading toward London, because they were so certain that an attack was imminent. They then put clues together about the Lancaster bomber that don’t make sense, like the fact that there’s no damage to the bomber that’s returning from a bombing run over Germany. They conclude that the Lancaster bomber must be the key to the attack on London. They make the difficult decision to shoot the bomber down, and when it crashes there’s a huge explosion. The main characters later discover that the bomber was a decoy operated by the Germans, and was on its way to do a sneak attack London. They also learned from Air Command that there was no Lancaster bomber scheduled to return that morning, and that they “saved the day.”
If anyone remembers the title of this book, I’d really love to read it again! Thanks!