The bottom started to fall out of the certified market 2-3 years ago. Prices actually paid have fallen some 30% just in the past year or two.
Now, a 5 year old IFR tourer in perfect condition, bought for £200k and with another £100k spent on it, might fetch £130k. An original model would have "merely" halved in value in 5 years. And this is aluminium - composites are worse affected (for whatever reason).
Planes have turned into cars. It's going to take a while for people to accept this though!
The only way to get one's money out of a plane is to do what one would do with a car: keep it for a long time, 10 years at least.