According to the Road Vehicles Lighting Regulations (RVLR), it’s illegal to cycle on a public road after dark without lights and reflectors. There have been quite a few changes to this legislation in recent years: the current version of the RVLR was published in 1989, and has since been amended multiple times, most recently in 2009.
To avoid major trouble with the law (and to stay safe), a white light must be showing from the front of your bike and a red light from the rear. The RVLR specifies minimum outputs for these lights and requires cyclists to fit reflectors too, so if you lack either of these things and are involved in a night time accident then it could be regarded as contributory negligence. But you’re unlikely to be pulled over by the police as long as the front and rear of your bike are illuminated.
This is backed up by Rule 60 of the Highway Code, which says:
“At night your cycle MUST have white front and red rear lights lit. It MUST also be fitted with a red rear reflector (and amber pedal reflectors, if manufactured after 1/10/85). White front reflectors and spoke reflectors will also help you to be seen. Flashing lights are permitted but it is recommended that cyclists who are riding in areas without street lighting use a steady front lamp.”