FIS
Back in the 80's I did the AOPA Ground Instructors Certificate, which granted priveleges to teach ground school for the (UK) PPL.
I believe that this qualification still exists 'on paper', but I don't know anywhere offering it, presumably as it is not recognised by JAR.
As far as I know, no specific qualification is required for teaching PPL ground school.
When I did the FI course, I was granted 75 hours credit towards the 125 hours ground school for the GIC.