Short answer:  There is no reason for, or benefit in, recording information about public fire hydrants in a fire risk assessment carried out in pursuance with Article 9 of the Regulatory Reform (Fire Safety) Order 2005 (as amended) (or equivalent legislation in Scotland or Northern Ireland).  

Long answer:
The dutyholder under the relevant fire safety legislation, for the building being assessed, has no control over public fire hydrants, nor do they have responsibility for their upkeep or maintenance. These duties fall to the water undertaking and the local fire & rescue service.   Indeed, unauthorised access to public hydrant pits is prohibited. This approach is supported, for example, in England, by sections 42 (6) and 42 (7) of the Fire & Rescue Services Act 2004 which state: 
(6) A person commits an offence if he uses a fire hydrant otherwise than—
•    for the purposes of fire-fighting or for any other purposes of a fire and rescue authority, or
•    for any purpose authorised by the water undertaker 
•      or other person to whom the hydrant belongs.
(7) A person commits an offence if he damages or obstructs a fire hydrant, otherwise than in consequence of use for the purposes mentioned in subsection (6).

Until the publication of BS 9792:  2025 Fire Risk Assessment, Housing. Code of Practice, the only reference to public fire hydrants in fire risk assessment guidance documents, including those issued by central government, was in relation to the obstruction of hydrants by parked vehicles. 

The fire safety maintenance check lists in the DCLG fire risk assessment guides include a recommendation to check that fire hydrant meter by-pass valves have been serviced by a competent person. Fire hydrant meter by-pass valves form part of a private fire hydrant system, and as such the valves and fire hydrants beyond the meter by-pass valve are the responsibility of the Responsible Person. That is not the case for public fire hydrants which are installed and maintained by local water companies and tested by the local fire authority.   

The commentary on clause 15.12  in BS 9792 states:  All systems, equipment and facilities for use by the fire and rescue service therefore need to be taken into account in the FRA, including evidence that they operate in accordance with their design specification, are tested and maintained, and that instructional signage for direction of the fire and rescue service is clear and appropriate.