Ok, not to worry. The next part of my confusion is where you mention it's the Avios needed for a redemption in WT.
I mean I understand that bit but the BA site advises that the price is based on the Avios needed for a redemption in the cabin you wish to upgrade from, so in either case here it's an upgrade from WTP to CW. Not from WT.
Sorry if I'm being a bit of a wally.