You could change the bulb to a 5/10w that would be classed as a side/aux light.
As for 'Guidance' on use of foglights - 'fog' is irrelevant, its visibility that needs to be seriously reduced, this could be fog, mist, spray, smoke.
Sadly the highwaycode actually guides people to use them incorrectly! It states they should be used WITH headlights, when by design they are only any use when your headlights light up the fog more than the road etc. The fog lights disperse light over a lower and shorter distance to light up what is closer to you, therefore headlights and fogs together are pointless.
The 'LAW' gives better guidance, its states you must use headlights when dark and/or when visibility is reuced, UNLESS you have fog lights. Denoting that you can drive without headlights on when you are using fogs (when visi is reduced).