Available headroom width

This sign means: Available width of headroom indicated. It is a warning sign. Warning signs are mostly triangular and tell you about a hazard ahead, so check your speed and be ready to react.

Official Highway Code caption: “Available width of headroom indicated.”

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