Police website:
Other lights - It is an offence to show a red light to the front of a vehicle (including a reflector) and a white light to the rear unless reversing. Once again these offences are dealt with by means of a £30 Fixed Penalty Notice. It does not matter if the lights are mounted inside or outside the vehicle, just that the light can be seen from the outside. This would also include neon lights fitted under or on the side of a vehicle and red LED windscreen washer jets. Green lights can only be fitted on Medical Practitioners vehicles.