This is something that a group of us did to death about a year ago. A dual carriageway is one where there is a divider that may not be crossed except in an emergency. So, for example, double white lines don't count since they can be crossed to overtake cyclists and other slow moving traffic that is travelling at 10 mph or less. However, hatchings bounded by solid lines do count since they can be entered but not completely crossed. The same with median grass strips since they cannot be crossed under normal circumstances (even via gaps in the grass intended for emergency vehicles to use or to permit contraflow for carriageway works etc.)
A dual carriageway with street lights less than the prescribed limit apart impose the default urban speed limit (30 mph in England) unless signed otherwise.
As from a few days ago, all new cars must be fitted with some of the most annoying tech going: Intelligent Speed Assist (ISA), which uses GPS mapping and cameras to detect the speed limit and restrict the vehicle to this. While you were able to permanently disable this, you now have to turn it off at the start of each journey and this is logged to be used in evidence?! Also mandatory is 'lane assist' that tries to keep you in the same lane unless you indicate and I suspect any cruise control system fitted must be adaptive. FWIW, 'funsters' have already shown issues with ISA and lane assist has already caused accidents. Funsters went around with tee-shirts that had speed limit signs on them that ISA 'considered' to be 'proper' speed limits. Lane assist, however, is worse as there have been several accidents where people have attempted to swerve onto the verge to avoid a head-on collision and lane assist has 'helpfully' guided them back into danger!