it's a bit of an "either way" incident. whilst it's clearly wrong to be driving on the wrong side of the road, your first obligation when pullingout is to look right to see that the road is clear. once you've done that, you are supposed to look the way you are driving as there might be a pedestrian (for example) in the road.
logic says the person who has strayed across the road is in the wrong, but I think it would be looked into if necessary. the Highways Agency should be aware of it as an accident black spot, and should look into some sort of traffic calming methods.