This is incorrect - the speed limit for a car on a dual carriageway is 70mph unless otherwise indicated by speed signs or a system of street lighting. Rule 103 of the Highway Code.

However, in England and Wales you have a quaint rule that says the 30mph limit applies to all traffic on all roads in built-up areas with street lighting unless signs indicate otherwise.

And a dual carriageway is a road with two or more running lanes in each direction separated by a central reservation. Without the reservation the road is a single carriageway even if there are two lanes in each direction.