Whilst flying back from GLA to LHR on BA1474 on Sunday, even though our takeoff and landing were carried out at night the cabin lights were not dimmed which I thought was strange. Am I right in thinking that it is a rule that pilots dim the lights in the event of an unscheduled disembarkment needing to take place? If it is a hard rule that must be followed then I am unsure how neither the captain or first officer realised that this had not been carried out prior to takeoff or landing.