I would consider it dangerous, careless or inconsiderate cycling, lights are there to assist you and to assist others, to not have them is to be inconsiderate to other road users.
For me Rule 68 fits the bill
You MUST NOT
ride in a dangerous, careless or inconsiderate manner
Law RTA 1988 sects 24, 26, 28, 29 & 30 as amended by RTA 1991
Rule 68
You MUST NOT
carry a passenger unless your cycle has been built or adapted to carry one
hold onto a moving vehicle or trailer
ride in a dangerous, careless or inconsiderate manner
ride when under the influence of drink or drugs, including medicine.
Law RTA 1988 sects 24, 26, 28, 29 & 30 as amended by RTA 1991
Rule 60
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.
Law RVLR regs 13, 18 & 24
http://www.legislation.gov.uk/ukpga/1988/52/section/28
http://www.legislation.gov.uk/ukpga/1988/52/section/29
.