It is, as stated in a previous reply, illegal to have blue lights showing to the front, side, rear or underside of a vehicle as it can be confused with an emergency vehicle. These lights are generally used by boy racers who enjoy wearing their base ball caps back to front. It is also illegal to use the low fitting driving lights or rear fog lights in normal weather conditions. What has surprised me though is the number of vehicles seen on the road nowadays with a failed head and side light on the same side. This is ignorant and very confusing as the vehicle could be mistaken for a motor bike.
Apparantly it is also illegal to use an amber flashing beacon unless you have a licence for it. I have yet to receive clarification on this so I may be wrong.