The law on side lights in the UK goes as follows:-
If you are driving on a restricted road that is illuminated by lamp posts not more than 200yards apart, then you are only legally required to use sidelights and not headlamps. (a 'restricted road' is, any road where the maximum speed limit is not greater than 30 mph).
At night, if you are parked on an unrestricted road (max speed limited greater than 30 mph), you are required to park on the nearside with (the minimum of ) offside sidelights illuminated, on to the front and rear to warn other road users that you are parked.
At night you are only aloud to park on the offside of the road, but still require sidelights, if you are on a one way street, but then you require sidelights on the nearside to be illuminated both front and rear.
When it comes to commercial vehicles, sidelights are required at night, to outline the vehicle to aid on coming traffic.
In days gone by, the use of head lights was frowned upon if you were following someone down the road, because you could be dazzling them from behind or creating a shadow, if your lights were brighter than the car you were following!