The Highway code

• ensure all sidelights and rear registration plate lights are lit between sunset and sunrise
• use headlights at night, except on a road which has lit street lighting. These roads are generally restricted to a speed limit of 30 mph (48 km/h) unless otherwise specified
• use headlights when visibility is seriously reduced (see Rule 226).

So, headlights not required at night, in built up areas. Let alone just a little overcast. Pisses me of people turning full headlights on as soon as the level of light drops slightly below "surface of the sun" level.

It's actually dangerous because on bumpy roads it can look to someone joining that they're being flashed out. I think because the contrast between the ambient light and the headlight is smaller that bit looking directly at the bulb stands out more and looks like a flash. When it's actually dark it doesn't stand out so much.