Just been reading about the law regarding bicycle lights and it seems that as well as a front and back light, a cycle must have red rear and orange pedal reflectors and as some bikes are now sold without pedals and hence without the legally required orange reflectors are the sort of.. not legal at night?
http://www.bikeradar.com/road/gear/article/bike-light-laws-in-the-uk-what-you-need-to-know-48568/
http://www.bikeradar.com/road/gear/article/bike-light-laws-in-the-uk-what-you-need-to-know-48568/