Are you a member of Which? If so, then contact them and see if they will take-up your case. If not, it might be worth joining and then asking them to take-up your case - this is the sort of thing they delight in for case studies and publicity.
Do you have legal expenses insurance, perhaps as an add-on to your car or house insurance? IF so, check what legal actions it covers - it might cover your legal costs to launch a claim against the car dealer for the (multiple) breaches of contract. Then write to the dealer threatening to sue.
You could also threaten to write to the local paper - check if they have a consumer column that might want to use this for an article.
Finally, you could just write direct to BMW UK or even the German parent, complaining that the attitude and service received so far must surely fall well below the standards they expect of their dealers and asking them what they intend doing about it before you feel compelled to write to the press - BBC's Watchdog would be an excellent reference to make in my opinion!