No need for anyone to wonder what BMW's Insured Warrany says about anything.
Summarised here:
https://www.motorrad-warranty.co.uk/Warranty#_WarrantyOverview
Policy wording (do though check that it's current) here:
http://www.google.co.uk/url?sa=t&rc...5AmhgGg-541wKoBSg&sig2=aEbJDHOHeON9D6480yZQCA
It's possible that bods are getting confused between the two-year initial manufacturer's warranty (which allows servicing outside of BMW's own dealerships, apparently) and the subsequent - insurance backed - warranty which seemingly doesn't. See page 16 of the PDF, which is the policy document, setting out the General Conditions. Do though check that the version held on the web is current.
That being said, UK consumer law * being what it is, I can imagine a circumstance whereby - in order to reject a claim under the insured warranty - BMW (or more likely, their insurer) would have to prove that the cause of the warranty claim was
as a direct consequence of mismanagement by the 'non-BMW person' carrying out the work. A tortuous process perhaps, especially when the unfortunate biker was backed by the full authority of UKGSer to do much as he pleased.
* The new Insurance Act will change things further