I thought they might have been hazard plates but the plates are just orange squares with no piccies or writing on them. Also, I've spotted them on inoccuous vehicles such as small white vans which blew my theory that it was maybe a long vehicle thing.
The transport of hazardous goods come under the ADR regulations (its 5 years since I took my test and I can't remember what ADR means ! I think its French, but might not be). Anyway, the plates on sft's web site (called Kemlar plates I think) are only used on tankers. All other vehicles carrying hazardous goods, whether a white van or 44 tonne artic, have to display an orange reflective rectangle on the front and back of the vehicle.