Let’s get this bottomed out.
Highway Code Rule 113:
You MUST
• 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).
Rule 114:
You MUST NOT
• use any lights in a way which would dazzle or cause discomfort to other road users, including pedestrians, cyclists and horse riders
• use front or rear fog lights unless visibility is seriously reduced. You MUST switch them off when visibility improves to avoid dazzling other road users (see Rule 226).
Rule 115:
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
Rule 226:
You MUST use headlights when visibility is seriously reduced, generally when you cannot see for more than 100 metres (328 feet). You may also use front or rear fog lights but you MUST switch them off when visibility improves.
So, in summary, headlights are a must at night where there are no streetlights. You should use them in built up areas at night and anywhere during the day when it is dull. You must use them anywhere when visibility is seriously reduced. Headlights should always be dipped unless they will not dazzle other road users. Also (my own particular bugbear) foglights must not be used unless visibility is reduced.