Fine for a dirty number plate?

[Matt UK]

Coming back over the dartford bridge I noticed a car which looked to have been recently cleaned, but the rear number plate had a layer of travel grime so thick it was impossible to read. It was clear that the driver had gone to the effort of systematically cleaning around the plate.

As I overtook him, I looked back to see that he'd done exactly the same with the front plate.

It's not hard to work out why someone would do this, but if the bib decide to take issue, what's the offence / points / fine?