On my car the rear fog light only works on Dipped beam not Main.
Cannot find any current legislation for the UK that requires this specifically. Following rear fog lights becoming mandatory in 1980 in the UK, various moves were made by manufacturers to ensure these high intensity lights were not accidentally left on, the current requirement of warning light on switch being not considered enough.
Having them only allowed on with dip beam is contentious as there may be rare situations where both main beam and rear fog lights to benefit those following, improve safety.
FWIW my 2018 car requires only that headlamps (dip or main) are on for rear fogs to function.
