Soldato
The 40 limit applies to LGV's on a dual carriageway that has no central reservation (that stretch has just a white line) effectively this makes the road to all intense & purpose a single carriageway as far as HGV drivers are concerned.
The definition of a single carriageway is what you have just described and that applies to cars as well as HGVs. Lots of people think that "dual carriageway" means two or more lanes in each direction, but it doesn't, it means that there is a physically separate carriageway for vehicles traveling in the opposite direction. Number of lanes is irrelevent. Physical separation can be anything from a grass verge or a curbstone up to concrete barriers or physically separate bits of road with nothing between them but a gap and a nice plummet into the river below.
Lots of people get caught out by this, doing 70 when there are two lanes in each direction with just paint, and more annoyingly on the few stretches of single-lane dual-carriageway there are far far too many idiots still doing 60.