BMW "warranty" is unbelievably complicated.
I think there are 4 different policies.
1) BMW Manufacturers Warranty. Lasts for 24 months and is backed by BMW in Munich. Is subject to regulation by European and English law, so they are not able to dictate unfair terms, such as dictate where the car is serviced. Cannot be invalidated, but they can allege that owner contributed to a fault, which is reasonable. Last time I saw a Warranty Manual, it was 3 inches thick. I bet it hasn't got thinner, and I don't believe they publish it anymore.
2) BMW (UK) Ltd 3rd year warranty. This is an extension granted by BMW in the UK, and seems to have similar (unwritten) terms to 1 above. (try as I might, I cannot find any written terms, apart from the cursory guidance in owners documents)
3) Extended warranty. This is as a result of a letter every owner will get when their warranty expires. I think it is an Insurance product. Not sure of the terms or who the Insurer is, but assume similar to 4 below. Has to run concurrently from the end of the BMW (UK) Warranty.
4) BMW Insured Warranty, available to all irrespective of service history provided less then 100,000 miles. Is "invalid" if the car is serviced outside of the network or modified from BMW specifications or used on track days.
However, so far we only had a single instance of BMW resisting a warranty claim on a modified car. They came to their senses once we provided them with information (that they already had) that it was a manufacturing defect.
I'd still love to hear from anyone who has had a clear warranty fault disclaimed.