Q400: A lot of cars have their fog lights on when it's not foggy, is it legal?

You must not use front or rear fog lights unless visibility is seriously reduced (see Rule 226 of the Highway Code) as they dazzle other road users and can obscure your brake lights. You MUST switch them off when visibility improves.

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.