[Considered1]

CAP is never going to be anything other than a trade in price, which is what a dealer will offer.

CAP = Car at Auction Price, no one is at auction when you trade in but the dealer thinks he is but any more for your car is coming out of profit on the other sale.

Windscreen price would generally be CAP + £2k-£4k (or your £3k Parkers value) depending on rarity of the car and others on the market.