The legal position during my time in blue is that you must not cause an oncoming car with right of way to alter course or to slow down.
The fact that everyone just seems to pull out into dual carriageways these days and expect people with right of way to move over just shows how bad driving has become in the UK - that does not even account for the bad road manners of doing just that.
You have to drive defensively as though everyone one else on the road is totally incompetent and watching the video I just cannot fathom what the Morgan driver was up to - he showed no self preservation for himself or his car.