Simply put, that stretch of road being only a single cars width isn't wide enough for a safe overtake of...well anything.
If the cyclist was in the gutter, and the van went for an overtake, the van would either be dangerously close to the cyclist (accelerating vehicles, especially large ones, create quite a strong undertow when overtaking, and you can get sucked in), or dangerously close to the oncoming traffic.
Therefore the sensible option is to wait until the road is clear.
Now unfortunately, mr cyclist here knows that 98% of UK motorists haven't got a fucking clue about how to drive, and are extremely dangerous and will do stupid stuff at the tip of a hat, so mr cyclist holds a central position on the road to force the traffic behind to drive safely. This is well within his rights so long as he is moving faster than 10mph.
Unfortunately, mr white van man knows nothing about laws and regulations, and tries to kill him.