This refers to the fire hydrant sign. Originally, UK fire hydrants showed two key measurements to fire crews: on the top of the "H", the diameter of the water mains pipe, in inches; below it, the distance from the sign to the access cover, in feet. More recently, fire hydrants have been labelled in millimetres (for the pipe diameter), and metres (distance to cover). Because the measurements are different orders of magnitude, there's no confusion over whether the markings are in imperial or metric units, so the units are not shown. (E.g. a 3" pipe would be the equivalent 75mm; no pipe would be 75 inches in diameter (far too big), nor 3mm (much too small). So this neat convention shows the essential info to the fire crew with the minimum "noise". For more on the conventional layout of UK fire hydrant notices, see this interesting page.