I think that all London roads should be single carriageway
Do you mean 'single lane' or 'single carriageway'? I suspect the former--'dual carriageway' is not the same as 'multi-lane' or 'two-lane'. It means that the carriageway has some form of central divider beyond just a painted line. I wouldn't agree that all streets should be single-lane in one direction, as that would mean no room for priority bus and cycle lanes. However, there are certainly plenty of streets in which the number of carriageway lanes should be reduced to one in each direction.
Do you mean 'single lane' or 'single carriageway'? I suspect the former--'dual carriageway' is not the same as 'multi-lane' or 'two-lane'. It means that the carriageway has some form of central divider beyond just a painted line. I wouldn't agree that all streets should be single-lane in one direction, as that would mean no room for priority bus and cycle lanes. However, there are certainly plenty of streets in which the number of carriageway lanes should be reduced to one in each direction.