I'm sorry, I have to chip in here.
When you buy a product or service in the UK, it has to be fit for purpose or a whole raft of consumer rights apply, which entitle you to various remedies. It's called "The Sale of Goods and Services Act 1982". Most people would not pay for a product that wasn't actually available (pre orders accepted) or that didn't work as described by the seller. The exceptions are generally investors, who accept the risk of non delivery for the chance of a return on investment (or are buying equity).
Tesla, of course, know about this and other consumer law, so have worded the description of FSD very carefully in their sales literature (a.k.a "The Tesla Web Site"). If you paid for it and are disappointed, you have my sympathy, but I for one didn't shell out the cash because I knew I could add it later, so why have the up front expense for something that doesn't add any, or much, value? But going back to Tesla and their no doubt extensive knowledge of UK consumer law, I suggest no one has a cat in hell's chance of a successful claim against FSD, because you can't make a claim based on rumour, Elon's tweets (which he can say are his own opinion) or your expectations of what "FSD" is or isn't. There is a slim chance you might win given a dictionary definition of "Full Self Driving" but couldn't Tesla say it's just a trademark and not to be construed as truth?
Auto headlights and windscreen wipers not working as expected might have a better chance. Class action, anyone?