Just to add a little more by way of explanation.
The BS476 test is just that- its a standard test of a foor assembly on the face of a gas furnace that allows door designs to be compared against a standard. The test in no way replicates a real fire- these are all totally variable and may place a door under much greater or much less fire loading dependent on the circumstances.
Many door manufacturers and manufacturers of intumescent products have submitted a whole range of door designs for test over the years and organisations such as BMTrada have produced specifications of door designs and upgrade techniques that have been found to pass the test. Your joiner is proposing to replicate one of these designs I guess. Every detail has to be carefully followed and some companies will then issue a copy of their test certificate that was granted for their door when it was tested.
Its far from a guarantee.
The rate at which timber chars is fairly predictable, its how the door may warp under the effect of the heat that is most likely to cause a failure around the perimeter.
Take a look at the ASDMA website
www.asdma.com