True story, I was in a dispute with KTM/Dealer/Santander on rejecting a bike that had gone all the way to the financial ombudsman, I had evidence of poor paint application at manufacture from an independent paint specialist who inspected the bike and gave me a report .... KTM and Santander disputed my claim with the Ombudsman as the specialist wasn't VAT registered and therefore wasn't a "qualified expert" .... the Ombudsman spoke to the paint specialist, he rattled off all his qualifications and she called BS on KTM/Dealer/Santander and made them take the bike back - I do think it's an "industry benchmark" to be able to at least say with some confidence that a garage is legit (rather than someones mate) as to make enough money to be VAT registered they will be assumed to be successful at what they do, but, turns out whilst thats what they want it to be as a benchmark, makes no difference to statutory rights in an Ombudsman's eyes and if it's not explicitly stated then it's just a "yeah but" that could be used to try to dispute a claim
Obviously a warranty is completely up to BMW to decide whether or not they will honour it, KTM didn't when I had proof of issue at manufacture, in fact in their case they told me to jog on without even speaking to me, just looked at pictures of corrosion and flaking paint and said "customer not looked after it" and there I was without any chance to argue anything after only 6 months of owning it from new (VAT registered garage or not - their warranty, their rules whether in the T&C's or not) so whilst it may indeed be a myth, I'd prefer to play it safe and at least use a well established independent, who is likely to be VAT registered if successful and hence problem solved