.....
Sorry .. as I never never use "sidelights" as you refer to them ..I think of them as "parking lights" ... the switch always goes straight from OFF to full lights... sort of assume everyone does the same .. 
I had this same conversation at work the other morning. One of the lads commented that as it was so light on his way in, he'd just been on side lights, to which I also referred to them as 'parking lights'. I'm with you Nige, lights on = dipped beam

(side lights might be marginally better than bugger all on mornings like today when it was piddling down although there were many without
)
I had an interesting chat many years back, over a pint, with an off duty police Sgt. He was a decent chap and very much "old school" ..ie . talk first to educate, book if you get nowhere. There were about 4/5 of us and the subject of "sidelights" came up .... and he stated.. quite publicly .. that he "loved em , greatest thing ever" ..... we were somewhat amazed and asked for further.. and his line of argument was this .......
The law requires you to drive on dipped lights whenever visibility is reduced ... simple. ( we all agreed)
If you put "sidelights" on you are admitting that lights are needed (why else put them on), therfore it is easy to nick you for not driving on dipped lights as you have admitted visibility is reduced.....
The driver on NO lights at all has a "line of defence" in that he can say that, in his opinion, visibility is not reduced enough to need lights, it is harder to nick him as it is a matter of opinion.
Obviously after "lighting up time" (30 minutes after sunset

) or in fog Mr PC has a far easier time ... but it is/was an interesting point that we could not really argue against ....
Be interested in Broocies take on it mind ...
