Oh dear you are confused aren't you. The speed limit for a car is the same on a motorway as for a dual carriageway.
There are miles of roads without speed signs as you are required to know the national speed limits for your type of vehicle.
A dual carriageway is defined as having a central reservation. Ther is no mention in the Highway Code of a need for a crash barrier.
I suspect this was a rogue flash, and your friend will not hear anything.
http://www.direct.gov.uk/en/TravelAndTransport /Highwaycode/DG_070308