£12k cash?
Ooft.
When my old Passat got to the eight year age rule in 2014 sold it for £1,300 with around 175,000 on the clock.
My current Passat is the same age, but age rule is now ten years, so intend holding on to it. But due to lockdown etc, it's only done 141,000. So a lot lower mileage than your own, but wouldn't even expect to get half of what you've been offered in cash. Of course, no doubt yours attracts a premium because of its green credentials, but £12k cash, £15k trade in?
Sure the dealer hasn't made a mistake? Seriously couldn't imagine a main dealer swapping a high mileage ex-cab for same car three years younger, with 160,000 fewer miles on the clock, for just £3k...