UK Car Repair Questions? Ask a Mechanic for Answers ASAP
Does the engine change RPM at all? It's pretty typical to be able to watch your lights on the bumper of the car in front of you, dim and then brighten again, as the charging system is most efficient at a higher rpm. If your car actually drops idle as if it is going to die, then I would suspect an engine issue (like a vacuum leak in the intake hoses) but otherwise it is relatively normal operation of the lights and charging system. You could always test the amperage output of the alternator, and load test your battery. Most shops will have a tool that can do this just by hooking up to the battery and testing both.