I've not read the entire thread (although I'd bet most posts are simply repeated from other threads).
I don't have a warranty. My Z is a 2nd car, if it breaks it doesn't need to be fixed immediately. If it was my daily driver I'd consider a warranty for convenience purposes. If however, I had a savings buffer of £5k, I don't think I'd bother at all. Yes things like engine failures etc are scary, but incredibly rare and in most cases I'd imagine BMW would offer some level of "good will". We're all enthusiasts and preventative maintenance should ensure that major failures don't occur (obviously you need to know what needs attention though).
As to whether it's good value or not:
1) There will be only one winner out of a warranties....that's the warranty provider and not the customer. If a warranty was "good value" when looking at the greater picture, warranty companies wouldn't exist (or they'd be more expensive). A warranty is not "good value" in the long run. Simple.
2) Most "it cost me £1500 to fix, warranty has paid for itself" posts refer to BMW labour costs etc. A reputable Indy wouldn't cost as much, so you'd need to take that into consideration.
That said, it all comes down to risk mitigation. If you'd like peace of mind, get a warranty. If you might not have a spare £2-3k at all times to cover the majority of issues that might occur and you need the car every day, get a warranty. But if you have a decent savings buffer or no immediate requirement for the car, a warranty might not be required.