This is what the 1964 Act has to say:
Continuous white lines
9. An authorised continuous white line along the centre of a roadway shall indicate that traffic must drive to the left of the line, and when on a stretch of roadway on which such a line has been provided a driver shall, save for the purpose of entering or leaving land or premises adjoining the right hand side of that roadway, drive to the left of the line.
Broken white lines on centre of roadway
10. An authorised broken white line along the centre of a roadway shall indicate that traffic must keep to the left of that broken white line unless it can be crossed without danger to other traffic or pedestrians, and when on a stretch of roadway along the centre of which such a line has been provided, a driver shall, save when he can cross the line without danger to other traffic or pedestrians, drive to the left of the line.
Parallel white lines on centre of roadway
11.—(1) Two authorised parallel continuous white lines along the centre of a roadway shall indicate that traffic must keep to the left of those lines, and when on a stretch of roadway along the centre of which such lines have been provided a driver shall drive to the left of the lines.
(2) An authorised continuous white line and an authorised broken white line parallel to the continuous white line, both extending along the centre of a roadway, shall indicate that traffic having the continuous white line nearest to its right shall keep to the left of the lines, and that traffic having the broken white line nearest to its right shall keep to the left of the lines save where they can be crossed without danger to other traffic or pedestrians, and when on a roadway along the centre of which such lines have been provided a driver shall drive to the left of such lines when the continuous white line is nearest to him and, when the broken white line is nearest to him, shall drive to the left of the lines save where they can be crossed without danger to other traffic or pedestrians.