Currently the law for the installation of fire signage is clearly laid out in the Health and Safety (Safety Signs and Signals) Regulations 1996. In the main this relates to Means of Escape, Fire Action Notices and Fire ID signage. These regulations endorse the relevant parts of British Standard BS 5499 (Safety Signs).
The requirement for a building’s signage should be included in their Fire Risk Assessment. If this is the case, then it is a legal requirement for the building’s signage to duly comply.
The viewing distance of a sign is the maximum distance from which the sign has to be comprehensible. When calculating size, it should be from the perspective of someone using the primary escape route.
Note, the viewing distance is calculated by the height of the graphical symbol, not the overall height of the signboard itself.