If your vehicles carry dangerous items you need to register with the Environment Agency.
For this purpose, dangerous items include: toxic gases; butane gas; propane gas; explosives; fireworks; other flammable items; strong acids; and radioactive materials.
You must register if you carry more than certain prescribed quantities of these items and if you fail to register you could face a fine.
If you are registered, your vehicles must display the relevant orange sticker or stickers listing the nature of the danger.
The regulations require you to appoint a Dangerous Goods Safety Adviser. This is a person who has passed an approved course of instruction. You need not employ them, they can be a consultant.
Further information is available from the Health and Safety Executive (HSE).
Source: Managing Your Company Cars; Colin Tourick