Ofcourse they will drop, it's only a focus at the end of the day. They do seem to be depreciating rather slowly however.
The problem is a £20k car can be nice or a total shitebag, there doesn't seem to be any difference in what people are asking, that's used cars for you in general I guess.

If you want some sort of affordable investment, get a 1M, they're going up in value all the time if that's the sort of thing you're trying to achieve.