its simple.

used cars have been going up since jan, now its around 25 - 30% since Jan.
not on all cars but on anything popular.

still a shortage of people buying new hence good late used cars not filtering down.

scrappage is removing lots of cars that would have been £500 to £2,000.

people are buying more used than last year so there is greater demand for less cars and prices going up.

The prices at the end of last year were artificially low, no point thinking about how much things cost then as those prices no longer apply.