... but it is also legal to use them independently provided that there is a warning light and they are angled down by at least 3 deg.
Only if visibility is poor. Highway code rule 94 and 201.
The UK regulations state that head lamps must be between 500 mm and 1200 mm from the ground (to their lowest edge) and anything else becomes an auxiliary lamp. Auxiliary lamps can be a max. of 1200 mm from the ground like head lamps but there is no minimum.
Auxiliary lamps can only be used in conditions of poor visibility (except when used as aux. main beam and go off when dipped). Poor visibility is deemed to be less than 100 metres visibility.
I'm wondering now whether to unmodify my car back to the original spec although I did find the "fog light" mode very useful when driving back from Rogerthorpe in a blizzard last year!
I'm not sure how you have modified your lamps, but if you do them the way I have explained before in the magazine, they will function exactly as the standard car i.e. aux. driving lights with main beam, AND you can use them with side lamps only in poor visibility conditions. So they are legal both ways. And additionally they will come on automatically if ever the dipped beam failed as it once did on me, so it is a safety feature too!
As an aside here, when the dimensions were in inches, the head light minimum height was 24 inches to the centre of the lamp, so with the newer metric figures, head lights now can be slightly lower than before.
Roy