Having dealt with BMW dealerships in the past I would suggest starting to do a bit of sniffing around right now
BMW will be keen to take an order before Jan therefore the bargaining chip is yours to use
Not sure if you have more than one BMW dealer near to you or not
In any event I would be inclined to ring BMW dealers far and wide and ask to speak to the teams dedicated motorbility advisor explain your situation and ask if they have any AP reduction offers now or coming up
Leave contact details and ask them to keep you updated
If successful then simply ask your local dealer to price match
Nothing ventured nothing gained
Also keep and eye on this sites dedicated deals/offers thread and also if you want you can ask the same thing there