When a building is designed, often only the position and rating of the fire door are included in the specification. In this case you often have the choice either to use the cheapest option available, or take the responsibility for ensuring the safety of the occupants and insist on certification by using third party accredited products.
Building contractors always feel the pressure to bring a job in on, or under, budget. Choosing cheaper untested products may save your customer money, but if a fire breaks out it will cost far more in lives and property. In the case of fire safety - peace of mind is worth a few extra pounds.
The fire door manufacturer gives clear advice for the installation of his door and the components that must be used. These installation instructions are included with the door. If you ignore these instructions, you could negate the fire door certificate, or worse still, be held responsible if lives or property are lost.