Centre line

A dashed marking used as a lane divider, or to indicate that a road, or a cycle path, has two-way traffic on it.

Centre line marking Parliament Square London

The Highways England standard, 'Cycle Traffic and the Strategic Road Network', states that 

Designers shall use centre line markings on two-way cycle tracks to reinforce the Highway Code which states that users should keep to the left. The use of centre line markings also helps to differentiate one-way cycle tracks from two-way cycle tracks.

The use of centre lines on two-way cycleways and paths is also recommended by John Parkin's 'Designing for Cycle Traffic: International principles and practice'. However the London Cycling Design Standards differs slightly, arguing 

Centre lines in cycle tracks are an option, to show two-way cycling and to help cyclists keep to one side, but they may increase cycle speeds and therefore increase risk to all users in partial separation scenarios.

Increased speed may be a risk, but centre line markings do provide much greater clarity to all users, especially to pedestrians who may not be aware that a cycleway or cycle path has two-way traffic using it in the absence of a centre line. Dutch two-way cycleways are consistently marked with centre lines.