what the heck is "autojumble"?

In the Mother Country, "jumble" is used to mean what we mean in the American Colonies by "rummage sale."

An "autojumble" is a specialized sort of rummage sale where people who care about cars, trucks, tractors and the like, assemble and offer for sale all manner of spare parts and tools and books and memorabilia and Lord knows what all else. I have a friend in Lincolnshire who attends such events for the primary purpose of obtaining tools made by Rolls Royce. Imagine that.