Headlight use while overtaking and being overtaken

When you are being overtaken, dip your headlight as the other driver comes past so that you don't dazzle them in their rear view mirror. 

If you are the one overtaking your lights should be on dipped beam. It's permissible to give a quick flash of your headlights before you start the overtaking manoeuvre to warn the driver ahead. Don't put your lights on full beam until you are past the level of the driver of the vehicle you're overtaking.

If the driver of the vehicle you're overtaking forgets to dip their headlights, adjust your rear-view mirror to avoid being dazzled. 

The Highway Code Rule 115 says:

You should also

• use dipped headlights, or dim-dip if fitted, at night in built-up areas and in dull daytime weather, to ensure that you can be seen
• keep your headlights dipped when overtaking until you are level with the other vehicle and then change to main beam if necessary, unless this would dazzle oncoming road users
• slow down, and if necessary stop, if you are dazzled by oncoming headlights