This is not a dual carriageway, either in terms of OpenStreetMap's mapping conventions or law - there is no physical separation of the lanes by a central reservation.

The OSM wiki for central reservations is here:
https://wiki.openstreetmap.org/wiki/Dual_carriageway

The legal definition in Schedule 6 Road Traffic Regulation Act 1984, which sets national speed limits for different classes of vehicle uses the following definition: 
“dual-carriageway road” means a road part of which consists of a central reservation to separate a carriageway to be used by vehicles proceeding in one direction from a carriageway to be used by vehicles proceeding in the opposite direction.
https://www.legislation.gov.uk/ukpga/1984/27/schedule/6/paragraph/2