The days I've been doing at PI through Drive Events don't require a CAMS licence. You don't need an extinguisher either, just a helmet. However, they bill it as a social event where timing is not allowed. Club events with timing require a CAMS licence, in addition to a mounted extinguisher and helmet. I'm not personally sure if a CAMS licence grants associated track insurance, I doubt it given how cheap they are and how expensive track insurance must be.