Well the guides are the Article 50 guidance. Therefore is it not case of justifying why your premises does not meet the same standards?
After all they are the only standards that can be enforced under the FSO.
No standard can be enforced under the RRO. it can only be refered to as an accepted standard that they would need to demonstrate they are acheiving or justify why they are not.
lets say an IO visits a office premise and is not happy with with the MofE signage, not because its not sufficient but because it's not complying to BS5499 . He could justifiably serve a notice to say that the signage needs to be updated to comply.
Now lets say the premise is a heritage site.
They provide you with a risk assessment that says, we have not provided signage in accordance with BS because of the adverse impact it would have on the heritage of the building. Instead, we have similair signage that clearly indicates the exit routes. Also, to mitigate non compliance to BS5499, and to avoid any confusion, we have posted staff at every point along any exit route, whilst the building is occupied, to direct people out of the building. This is to mitigate the following statement in BS5499
It is
recommended that all signing systems should conform to the recommendations of this Code, so as to
minimize the risk of confusion for the public.
Their solution would be non compliant to BS but, in my opinion, would be satisfacory. remember british standards state:
As a Code of Practice, this British Standard takes the form of guidance and
recommendations. It should not be quoted as if it were a specification and
particular care should be taken to ensure that claims of compliance are not
misleading.