Why have a sign in sheet? In November 2007, a warehouse in Warwickshire UK collapsed during a fire killing 2 firefighters who were searching the building for 2 unaccounted for members of staff. It turned out that the 2 staff had gone off site and not signed out.
Not all businesses have emergency wardens, not all visitors are supervised. A sign in sheet keeps a record of who has entered and left the building, and can be used as a roll call register in the event of an evacuation. It doesn't matter if its a building or a construction site.
I'd suggest that Health and Safety at Work (General Risk and Workplace Management) Regulations 2016, Section 14 Duty to prepare, maintain, and implement emergency plan is the Regulation that would cover this, and the sign in sheet is part of the risk mitigation.