Unless it a new commercial building, were the Architect would conducted the risk assessment, and would submit to BC for approval. The category would be selected by the architect upon his knowledge of the building. As most commercial buildings since the 7o`s, were fitted with fire alarm systems, designs are mainly new builds, extensions, updates and change of purpose.
I am sorry Tazz but in my experience the Architect would never carry out the risk assessment. It is always someone with specialist knowledge, building control are always consulted and the building insurers are also consulted.