the answer depends on the installation type, is this simply for the building occupier to react to, reset test etc or is this sited for 3rd party information like the fire brigade to identify the location of said fire.
Anyone installing, designing and/or signing off a system should by fully knowledged in the relevant standards, where a specific height isn't given in regulation then you must use a logical approach which reflects the installation, occupants and any 3rd party that may need to refer to the panel.
Questions about your part in this query are therefore relevant and an idea of the installation etc mentioned above could change the advice given given that this is a safety critical system as the industry is heavily regulated.
If this is not a basic domestic install for which having a fire panel would suggest it isn't then either you are installing this under instruction of a designer who will tell you what the mounting height is as they will have taken all the above into consideration, if you are designing, installing and signing this installation off then I would have expected the query to be worded different and reflect you cannot fine the relevant info in the BS 5839 - this is not covered by the BS 7671 and you need to be very careful if you haven't taken the relevant courses, you should at minimum be having the install designed and signed off by a competent person if you wish to do the install yourself.
I personally have my bigger installs designed and signed off by others, I do the install and go on the courses for the fire alarm manufacturers to protect my arse in future maintenance and testing... gone are the days when you could simply pick up a fire alarm panel and install it, you now need to liase and local fire services, building control etc in anything bigger than a simple domestic install.