No joke, really did see the numberplate PEN 15 on the M25. Furthermore, I remember it was in italics, which made it look even like the word. Maybe you're getting mixed up with PEN 1S, which is clearly worse.
He was indeed, I was coming off that slip road that goes west > east onto the bit that goes south > north towards Dartford Tunnel. He was coming from the Kent direction like a Bat Out Of Hell.