I have had BMW UK replace a car before. one tip be patient and polite even if you are p***ed off. I think my attitude is what in the end made BMW UK hand over another car. Simply put I bypassed the call centre/customer services and wrote a very long email detailing every issue with the faulty vehicle. said email was then sent to the BMW UK General Manager and also their UK/EMEA Customer service manager, these two corporate big wigs obviously passed my email down the chain (bit of delegation is a good thing) but point is they only passed it a level or so down and my case was handled by a senior BMW UK mgr. lets just say everything moved VERY swiftly after that especially as I kept the UK/EMEA big boss cc'd on every communication
paid off in the end though. be polite, firm and professional and it gets results!