Merbau is classified as a bushfire-resisting timber in Australia because it has fire characteristics similar to timber treated with a fire retardant. This doesn't mean it won't burn, just that the heat released when it does burn is within the limits specified in Australian Standard 3959-2009.
British Standard 476 has a number of parts. The part dealing with non-combustibility is Part 4 which specifies a test to differentiate between non-flammable materials, such as plaster-based wall linings, and materials which undergo flaming combustion, such as wood products. It is most unlikely that any wood product would achieve classification as a non-combustible material as defined in BS 476: Part 4. Of course, "non-combustible" materials may still have undesirable fire qualities in that they may melt, shrink, collapse or change their nature in some other way. However, by definition they will not contribute to the fire load.
For more information about the design, construction and maintenance of timber in bushfire-prone areas, download the free WoodSolutions guide – Building with timber in bushfire-prone areas.AS-3959-2009, British Standard 476, BS 476, merbau, bushfire-resisting, bushfire, BAL