The difference between trunk roads like the A30 and a single-carriageway road or even one of those urban 40mph dual carriageways, is that on trunk roads there is an expectation among motor traffic not to have to slow down. The slip-road system is at least partly responsible for this. The aim of a slip road is to keep all traffic on the road moving at cruising pace, you don't have to slow down for traffic leaving or joining because they are provided the means to accelerate/decelerate before joining/after leaving.
Whereas on single carriageways I think it is accepted that you will have to slow down for bends, junctions, hazards, ... so traffic is more prepared to have to slow down if necessary to overtake cyclists, as well.
FWIW I can completely see Regulator's point. The prevailing attitude does annoy me and I have found myself starting using the 'safety in numbers' quote to ignorant but well-meaning people who display the 'idiot motorists make roads dangerous for cyclists => therefore don't cycle' attitude, and they've sort of gone - 'oh yeah, I suppose - you've got a point there'. However, I think there is a limit to how far that theory stretches - and I think the line is drawn at trunk roads with slip roads where there is no other reason for traffic to have to slow down to cyclist speed... than a cyclist.
The ideal, obviously, would be for Regulator to persuade enough people to cycle on the A30 that motor traffic gets used to them, and just accepts it as fact that you get cyclists on this road and that you will occasionally have to slow down for them. But have a look at it statistically, I don't think it's ever going to happen, realistically, is it - the critical level just isn't going to be reached.
I think it would be nice if it were mandatory to build a good quality, tarmac cycle path next to all new dual carriageways that are built.