Keep left

The keep left sign, a blue circle with a white arrow pointing down to the left, means you must pass to the left of it. You usually see it on a traffic island or bollard in the middle of the road. When the arrow points down to the right, keep right instead.

Official Highway Code caption: “Keep 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.