The point being that the 20 weeks starts from the date your order was put onto the system. 
I would say exactly that. When VW accept your order the clock starts ticking. If they take orders from BW13 they should be able to deliver in BW33 as long as the car is actively on sale 3 or 4 weeks before this expected delivery date.
Benfield were approaching p/x pricing in a slightly different way when I was there this weekend. They were saying:-
"right now your car is worth £1900 more than your settlement figure. We will guarantee that equity in the car at point of delivery, and if true p/x price in Nov or whenever allows for a bigger equity we will give you it, but it won't be less than £1900, whatever happens".
First time I have never been offered a fixed p/x price at point of potentially ordering (which may seem low for anticipated value in a few months time etc). With that logic they shouldn't need any assistance from VW for depreciating p/x value as they are giving you less every month that passes and so protecting themselves from that depreciation.
A dealership that grabs your p/x at week 20 to sell and puts you in a courtesy car until your new one turns up shouldn't need any depreciation money from VW, perhaps assistance in provision of the courtesy car (will the courtesy car be supplied centrally by VW uk, or from the dealership's own fleet?