I'm going through the process of making a MCIA format complaint. I have written to BMW Motorrad UK and was not satisfied with their answer (they are prioritising production over updating existing). I got on to the MCIA and they advised me how to proceed.
I had to write to the dealer as step 1, so I rang them (Cotswold Motorrad) for the dealer principles email address. I have a very good relationship with Cotswold, I'm on my 3rd (of 7) new GS with them. They were aware of what I'm doing but, like all dealers, they have no way of helping until BMW supply the headlight units.
What they did tell me was that BMW will be writing to everyone affected by the delay and offering a £500 voucher to be spent at the dealer as an "apology". The letters should be coming out this month. The £500 does not effect your entitlement to the updated headlight.
As soon as Cotswold Motorrad reply to my letter I can submit my complaint to the MCIA - I have already completed step 2, writing to the manufacturer, and can move on to step 3, "If still not satisfied, you may decide to refer your complaint to the National Conciliation Service (NCS) who are certified by CTSI to offer Alternative Dispute Resolution (ADR). NCS are independent and impartial and the ADR services offered are free of charge."