turn on your headlights
The headlights could have bad bulbs or a bad fuse; the two sets of lights are on different circuits.
check the fuses - there is no fuse for the headlights.
First check your fuses if none are blown then check the switch.
Turn on headlights/parking lights and adjust dimmer control
Because it is British.
If you turn up your interior dashboard light settings, then the radio display will be brighter too. The radio dims to the setting for the dashboard when the lights are turned on.
Fuse for parking lights probably blown.
its probably something to do with the wiring system
Because the power for your headlights is pulled from your battery. When the dashboard lights dim after turning on the headlights the dashboard lights aren't getting as much power as they were with the headlights off. With the motor running or off? The headlights draw a lot of current, but should not visibly affect other systems. There could be a problem with a worn out battery, an alternator that isn't charging correctly, or you have a bad earth between the battery and the chassis. Check the large cable connection between the battery (usually negative these days) and the bodywork, or between the engine block and the bodywork.
Check headlights and dashboard lights first. If no/minimal power for the lights then replace battery.
usually this flickering affects the headlights to and is caused by a bad alternator