Sadly, it isn't always straightforward. The 730mm (or 760mm on lines cleared for 373/2) lateral / 915mm vertical is measured in the plane of the rails, e.g. taking account of the cant, whereas (most) platform front walls are vertical with an oversail on (exceptions to most of these statements exist somewhere).
If NR "updates" a platform it comes under some considerable scrutiny if it tries to scrimp, and one of the other rules that is quite hot is the so-called "z" value, the overhang of the coper edge relative to the front wall, which is intended to be a "roll-space" provided for safety reasons.
The book dimension is 300mm and NR can generally justify an existing 200mm, but if it reduces a value already below 200mm it runs into trouble.
Put all this together and add in that the normal concrete oversail won't deal with "z" > 400mm, then a great many platforms would need the front wall re-building to allow the height to be made compliant. This is not a short, simple, cheap exercise on potentially 1,000* plus platforms, when carried out next to a running line.
* 2,500 stations nationally, average of two platforms per station, assuming 20% of these non-compliant (which in my experience is an under-estimate)