93: You MUST

• use headlights at night, except on restricted roads (those with street lights not more than 185 metres (600 feet) apart and which are generally subject to a speed limit of 30 mph)
• use headlights when visibility is seriously reduced (see Rule 201).
• ensure all sidelights and rear registration plate lights are lit at night.
  Laws RVLR regs 24 & 25 & RV(R&L)R reg 19

94: You MUST NOT

• use any lights in a way which would dazzle or cause discomfort to other road users
• use front or rear fog lights unless visibility is seriously reduced. You MUST switch them off when visibility improves to avoid dazzling other road users.
  Law RVLR reg 27

95: You should also

• use dipped headlights, or dim-dip if fitted, at night in built-up areas and in dull daytime weather, to ensure that you can be seen
• keep your headlights dipped when overtaking until you are level with the other vehicle and then change to main beam if necessary, unless this would dazzle oncoming traffic
• slow down, and if necessary stop, if you are dazzled by oncoming headlights.

96: Hazard warning lights. These may be used when your vehicle is stationary, to warn that it is temporarily obstructing traffic. Never use them as an excuse for dangerous or illegal parking. You MUST NOT use hazard warning lights whilst driving unless you are on a motorway or unrestricted dual carriageway and you need to warn drivers behind you of a hazard or obstruction ahead. Only use them for long enough to ensure that your warning has been observed.
Law RVLR reg 27