How close to the kerb should I cycle?
You are already riding considerately by the sounds of it which is great.. I can only advise though on what I used to do. The Highway Code is somewhat vague regarding exact rule for cyclists.. However you ARE entitled to be on the road and drivers ARE obliged to give you plenty of room when overtaking you. 

As a racing cyclist (a few years ago now) I would always ride about 2 feet away from the curb, especially on roads that were wide enough for it not to be a problem. This would allow me to clear the drain covers and any recesses they were in, whilst also avoiding the stones &amp; debris which is often laying at the roadside.. I don't think in all my years of road training that I ever had a real problem with drivers.. Well apart from the selfish few that believed that I should just not exist on their road.. But you'll always get those ☺️

Personally I wouldn't advise riding towards the middle of lanes as I know of folk who've been hit directly from behind doing this, as they've just not been seen at all.. as hard as that is to believe. 

IMO I'd rather be passed closely than hit directly from behind. This has even happened to my brother as he was stopped at traffic lights on a large motorbike. 

On your dual carriageway if there is no cycle track/lane just be careful and aware.. Ride about 2 feet from the curb, wear bright clothing and use a red strobing rear LED light, even in the daytime! Also as the roads are so busy and damaged these days a small camera may be worth considering. Not only would it be helpful in the event of an accident, but it will also assist with any claims to the council for damage to your wheels &amp; tyres etc travelling on todays broken roads.. 

I have been refunded on a couple of occasions after providing pictures to the council of damage to my wheels and tyres due to hitting large water filled pot holes in the wet. 

Take care and ride safe