Large vehicle crossing double white lines

[Wollemi]

Just this evening I followed a coach going along a B road (B6161 north of Otley). The width of this vehicle was such that it was necessary for it to cross the double white lines in the centre of the road - by several feet in places.

What is the legal situation here? Is it legal to cross the double whites if the width of the vehicle is such that it would be impossible to drive along it otherwise.
If not legal, whose responsibility would it be - the driver? Or the coach company for sending the driver along such a route in a too wide vehicle.
(Note, this road is not subject to a width restriction)

Later on, the same coach also had to drive over a mini roundabout (rather than around it) - is this legal?

[Wollemi]

So is the law being a bit of an ass here?

If it is dangerous to cross the double white line by choice, then surely it's just as dangerous if you have no choice? The vehicle coming in the other direction around the blind bend doesn't care how you got onto the other side of the road?