Unfortunately the amendment regs only make the no-entry sign enforceable. They don’t allow you to put one up if there’s no traffic order. Arguably the statutory requirement implicit in creating a dual carriageway might be sufficient to meet the need for an “Act, order, regulation, bylaw, resolution or notice” that restricts the road. However, other signs for dual carriageways have an explicit exemption from the need for an Act, etc., thereby implying that a dual carriageway status alone does not confer sufficient powers for an authority to erect these signs without something more explicit.
I wonder though if an authority could just make one blanket order (or even just pass a “resolution”, since that’s in the list) for all its dual carriageways (and their slip roads) that then allowed them to erect no-entry signs wherever necessary.
For signalled junctions, the bigger problem is turn-left and ahead-only box signs attached to the signal heads to direct traffic the correct side of a splitter island. These also need a traffic order, but are often put up without one in my experience. For the opposite direction, a no-entry sign without an order is perhaps a lesser sin!