The standard for signal spacing appears to be GK/RT0034 (http://www.rgsonline.co.uk/Railway_Group_Standards/Control%20Command%20and%20Signalling/Railway%20Group%20Standards/GKRT0034%20Iss%204.pdf). This sets the minimum distance from the first caution signal (single-yellow for three-aspect signalling, double-yellow for four-aspect) to the stop signal based on the principle that if you're running at the permissible speed and start braking at the first caution signal you should stop in time. It also restricts the positioning of single-yellow aspects in four-aspect areas and imposes a maximum signal spacing (usually 1.33 times the minimum, but can be up to twice the minimum). This suggests that you can get a rough idea of the signal spacing from the linespeed, but you'll need some route knowledge to avoid accidentally stopping too early.