I'd have to wonder whether a refueller who is so dozy as to not notice he'd replaced an Avgas truck with a Jet A truck *and* not noticed the different smell & feel while pumping, would notice a little placard next to the cap.
As for Lydd: What about the Kingair I fly? It's approved to use Avgas instead of Jet A**. If I need avgas in it for some reason and the placard specifies Jet A are they going to refuse to supply the fuel?
**Yes, I know there are maintenance penalties but that's not the point.