One of the problems regarding DRL regulations AFAIK, is that the IVA regs. differ from the vehicle construction and use regulations. This allows more freedom to car manufacturers to add bells and whistles to their latest models whilst IVA regs only get updated every few years or longer. My daily driver has DRL's that dim when the sidelights are turned on, whilst in the IVA regs it says DRL's must be turned off when the headlights are turned on, with no mention of side lights.
It could be argued that the IVA regs are more relevant to classic car modifications as these are the regulations that we would have to comply with in the event that the DVLA felt that an IVA test was required.
For anyone interested, the IVA regs can be found at https://assets.publi...er-vehicles.pdf
and section 22 deals with DRL's and sidelights.
I think if we stick to the obvious reasons for using DRL's then we shouldn't go far wrong.
1/ To give extra warning to other drivers and pedestrians in the daytime the DRL's must be very bright.
2/ At night time this extra bright light could cause dazzle to other drivers or even obscure your own cars indicators etc.
3/ Therefore, dimming the DRL's when the side lights are on and turning them off altogether when the headlights are on, would seem logical.