British number plates are there any rules?

nvingo

The only circumstances I've seen something like, is on the back of a pick-up truck. The plate hangs from the tailboard, so whether it's closed or open (as it would be if a longer load overhung the back of the bed), the plate is still vertical and readable from behind. Conceivably it could rise up to horizontal due to aerodynamics.
I don't think that's exactly what the OP saw though.
I've also seen plates where the white or yellow background colour is heavily black-tinted which I suspect is supposed to render them unreadable to a camera, though I wouldn't risk my licence to it actually working.

Bl4ckGryph0n

Agreed. I just keep to the speedlimit

maxwell

And if this joker gets caught he gets done for Perverting the course of Justice which is taken very seriously by the courts, 99% of the time resulting in jail time.

Naaktgeboren

I hope he does get caught and the fact he was maybe unknowingly heading in the direction of Kent Police's Tactical Operations centre (just off of the M20), I think he has a good chance of getting nicked.  I don't think I've ever cleared that roundabout without seeing Police vehicles either on the way in or out.