check this link -> https://www.osha.gov/SLTC/etools/evacuation/floorplan_demo.html
BS:5499 (4.6 – mounting height), says
a) signs above doors or open spaces should be mounted* between 2 m and 2.5 m from floor level, measured
to the base of the sign and be sited as close to the centre line of the escape route as practicable.
* This mounting height is intended to ensure that signs can be readily seen, e.g. over the heads of people.
b) signs sited on walls should be mounted* between 1.7 m and 2.0 m, from floor level measured to the base of the sign.
* This mounting height is intended to ensure the signs are within the immediate field of vision.
c) mounting heights greater than 2.5 m may be used, e.g. in large open spaces or for operational reasons, but care should be taken to ensure that such signs are both conspicuous and legible.
d) Signs should be sited at the same height throughout the escape route, so far as is reasonably practicable.