The Wunderlich version does look quite expensive. I think that about most things nowadays but £200 seems a bit steep and you only get one! You can get this or similar from the classic bike shop, which I am sure does not give the same illumination and does not come with the right canbus wiring but is around 10% of the cost if you are looking for form over function. It may well fry the ECU for all I know. Chris
I have been retired a few years now, but I am 99% sure there is no problem. If the motorcycle was being ridden without the headlight on, then yes, not legal, but it's not being so ridden.
Back in the 1970's & 1980's BMW supplied an accessory kit for the twins of the day, with a spotlight on the offiside of the headlight, that came on with "High beam", & a fog-lamp to the nearside that only illuminated with dip-beam.