No left turn

The no left turn sign, a red circle with a black arrow turning left struck through by a red bar, means you must not turn left at the junction ahead. Signs with red circles are mostly prohibitive, so carry on and find another way to your destination.

Official Highway Code caption: “No left turn.”

Listed under “Signs giving orders” in The Highway Code: traffic signs (GOV.UK). For every sign in more detail, see the Department for Transport’s Know your traffic signs.