The requirements are laid out in the Road Vehicle Lighting Regulations 1989 (as amended). In brief they are:
Front Light
A white light, positioned centrally or offside, up to 1500mm from the ground, aligned towards and visible from the front. If capable of emitting a steady light, it must be marked as conforming to BS6102/3 or an equivalent EC standard. If capable of emitting only a flashing light, it must emit at least 4 candela.
Rear Light
A red light, positioned centrally or offside, between 350mm and 1500mm from the ground, at or near the rear, aligned towards and visible from behind. If capable of emitting a steady light it must be marked as conforming to BS3648, or BS6102/3, or an equivalent EC standard. If capable of emitting only a flashing light, it must emit at least 4 candela.