Apparently, the part ex value of Audi's, Vw's, Mercedes and BMW's has plummeted by between 4 and 5000 pounds according to the car dealer my son is getting his new car from. ( Not any of those I must add).
That's an utterly meaningless claim. A 6-month old tricked-out Audi R8 will have depreciated by 10s of thousands but a 5 year old A1 will be worth more now than it was 2 years ago.
My son recently had his 14 plate Vauxhall Corsa stolen and the insurance company wrote it off. 3 years ago it cost about £3k and the insurance company just paid him £5100 net of the excess. Good second-hand cars - particularly the brands mentioned by the OP are like hen's teeth and fetch very good prices.
The dealer is taking the piss - shop elsewhere!
Also - try webuyanycar.com - I've sold a few cars via that route and got way more than a dealer would p/x for. Painless transactions too.