How realistic or unrealistic is it to expect 10-15% price drops when this is all over?
The Das Welt screen price has about a 35-40% markup over the tradein/purchase price.
Some of this covers the fact that they are selling out of swanky dealerships which cost a fortune to run, plus I'd imagine there is a cut for VWUK for it to be sold under Das Welt with a VW UK warranty.
I'd say you'd be lucky to see them budge 5%.
Cash won't make any difference, they don't care about cash. Finance is where they make their money.
The trick you might want to see is if VW are offering any deposit contribution on Das Welt used cars on PCP. If they are, you can get that by taking PCP and exiting the deal in the first couple of days, paying the outstanding balance off with cash - assuming you have cash - and you get to keep the PCP contribution VW made.
If you need to finance it, you are better off steering well clear of VWFS offers on Das Welt - those are eye watering interest rates. Get yourself a cheap car loan from another lender - check out Money Saving Expert for the best rates.