Yes, you must wear a hard hat on a construction site. This is a requirement under the Construction (Head Protection) Regulations 1989.

Safety helmets (hard hats) protect the head from falling objects, such as bricks, tools, and debris. They can also help to protect the head from electrical shocks and contact with hot surfaces.
Construction sites are inherently dangerous places, and hard hats are one of the most essential personal protective equipment (PPE) that workers can wear. By wearing a hard hat, workers can help to reduce the risk of serious head injuries.
There are different types of hard hats available, and workers should choose a hard hat that is appropriate for the hazards that they are exposed to on their particular construction site. For example, workers who are working at high elevations may need to wear a hard hat with a chin strap to prevent it from falling off.
Workers should also make sure that their hard hats are correctly fitted. A hard hat that is too loose or tight will not provide adequate protection.
In addition to wearing a hard hat, workers should also follow other safety precautions on construction sites, such as safety glasses, gloves, and boots.
By following safety precautions, workers can help protect themselves from injury and stay safe on construction sites.

Ensure safety on your building site. Shop for hard hat signs at The Sign Shed.