As they don't specifically state Km, the authorities could quite easily say they are groats (or whetever). And as they are simply distance, perhaps location markers, there is nothing to enforce. The southbound ones have an 'A' and the northbound a 'B', so I guess they are location markers for the emergency services. "There is an RTA at A196.5" for example.
Del
Del, you summise correctly.
Legally, they are the same as the plastic markers but more prominent and easily viewed at high speed by the Emergency Services.
A note to Perdu, marker posts are at 100m, have you measured them? They vary between 80-120m generally, although on the Stafford section of M6 J12-15, there are some 150+m apart (I know, I put them there!!). Over 200 miles the maximmum 'creep' can be about a kilometer, sorry, 5/8ths of a mile.
When these were introduced, the location was governed by a point on the central reserve and then transpossed to either side of the motorway so it could form a cross-section.
The 'gap' would enlarge and reduce depending on the horizontal curvature and side of the carriageway.
It is very rare to set these out with a measuring wheel nowadays. These days you have the element of an additional 'measurement creep' as everything is 'set out' horizontally and not catering for gradients.