Turn left

This sign means: Turn left (right if symbol reversed). Signs with blue circles but no red border mostly give a positive instruction: they tell you what you must do.

Official Highway Code caption: “Turn left (right if symbol reversed).”

Listed under “Signs with blue circles but no red border mostly give positive instruction.” in The Highway Code: traffic signs (GOV.UK). For every sign in more detail, see the Department for Transport’s Know your traffic signs.