Rule 220

Powered vehicles used by disabled people. These small vehicles travel at a maximum speed of 8 mph (12 km/h). On a dual carriageway where the speed limit exceeds 50 mph (80 km/h) they MUST have a flashing amber beacon, but on other roads you may not have that advance warning (see Rules 36 to 46 inclusive).

Law RVLR regs 17(1) & 26