It depends entirely on the road, the widths of which vary, there may be parked cars, the other lane may be quiet or busy.
It does seem that much of the chat in this thread is based on a misinterpretation anyway, but I don't see what is gained by keeping in the middle of the lane when the left hand side of it is safe. I think, in practice, it will make cyclists more in danger as it will enrage drivers who may be late etc. And yes, maybe they shouldnt be enraged, but this is a Real Politik situation.
And I am a cyclist and a driver. In fact, driving comes below the train and walking for me, so I'm not remotely a petrol head, I just dont see the benefit in winding people up.