As it contains a substance that can cause permanent damage to anyone who comes in contact with it if it breaks and the mercury escapes and the OP has stated in their opening post "I have employed couriers who are certified to move hazardous materials " then I'm assuming that the OP is telling them. 

I'm saying that I'm surprised that Parcelforce accept it, I wouldn't have thought that they would.

Key Dangers of Mercury:

• Elemental/Metallic Mercury (e.g., in broken thermometers): The main risk is inhaling vapour, which can cause cough, chest pain, tremors, headaches, and permanent brain or lung damage.