Legal Requirements
In the UK, bikes must conform to the pedal cycles (construction and use) regulations and the road vehicle lighting regulations. To be used at night, a bike must have:
One steady, fixed white light, marked BS6102/3 (or equivalent), positioned centrally or offside, up to 1500mm from the ground, aligned towards and visible from the front.
One steady, fixed red light, marked BS3648 or BS6102/3 (or equivalent), positioned centrally or offside, between 350mm and 1500mm from the ground, at or near the rear, aligned towards and visible from behind.
One reflector, coloured red, marked BS6102/2 (or equivalent), positioned centrally or offside, between 350mm and 900mm from the ground, at or near the rear, aligned towards and visible from behind.
Four reflectors, coloured amber and marked BS6102/2 (or equivalent), positioned so that one is plainly visible to the front and another to the rear of each pedal.
There are exemptions for older bikes; pre-1990, the light output from the front light to meet the required standard is much lower, and pre-1985, the pedal reflectors are not required. Note that there is no exemption for step-in (or quill) pedals1, which are popular on mountain bikes and almost universal amongst racers and tourists. No pedal reflectors, no night riding - legally, at least.
Flashing-red rear lights are also illegal in the UK, despite being between three and five times more visible. The lighting regulations, which cover both these and the pedal reflectors, are under review, so this may change.
In addition, various regulations pertaining to the sale of bicycles have led to the fitting of white front-reflectors (mainly because cyclists often fail to fit lights) and spoke-mounted wheel-reflectors, which are worthless in almost all situations. They can break spokes and unbalance the wheels. These are not covered by the lighting regulations. Remove them as you see fit.
http://www.bbc.co.uk/dna/h2g2/A686784