The thing about losing the 20% VAT as soon as you buy the car is flawed and just a trick of the trade used by dealers to pay less for part exchanges. A car is worth what someone will pay, and nearly new and short-supply cars are and always have been priced near or sometimes above the original retail price including VAT (and Car Tax).
I personally expect M3W values to harden slightly as the new car sales decline and faults are fixed on earlier cars, but they will be around £10K below that of a new car because of the short lead times these days and the "I can choose the exact spec myself" premium.
For the trad Morgans there is a market perception that there are still long waiting lists and great value retention has been long established but I bet if you try to trade in a nearly new car you are offered a lot less than once might have been the case, again because a new car is available at quite short notice.
