Thank you for that ... but I'll still point out that "most" people would describe "fog" more in terms they use everyday .. and the highway code is just another source ??
So, to a non-military trained observer ....
Driving in adverse weather conditions (226-237) : Directgov - Travel and transport
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 (see Rule 236).
236
You MUST NOT use front or rear fog lights unless visibility is seriously reduced (see Rule 226) as they dazzle other road users and can obscure your brake lights. You MUST switch them off when visibility improves.
Yet another anomaly ???