Rule 173 of the Highway Code states:

Dual carriageways. When crossing or turning right, first assess whether the central reservation is deep enough to protect the full length of your vehicle.

• If it is, then you should treat each half of the carriageway as a separate road. Wait in the central reservation until there is a safe gap in the traffic on the second half of the road.
• If the central reservation is too shallow for the length of your vehicle, wait until you can cross both carriageways in one go.

Before emerging right onto a dual carriageway make sure that the central reservation is wide enough to protect your vehicle. If it's not, you should treat it as one road and check that it's clear in both directions before pulling out. Otherwise, you could obstruct part of the carriageway and cause a hazard, both for yourself and other road users.