Not sure why but the value of .co.uk domains are definately going down. I would have thought they would be going up, as more and more domains are registered they should be more valuable for decent ones as its harder to get a good domain.
Either people don't understand the value of a good domain, or just think people are overpricing them, as they know that when they where first registered they where very cheap.
The average person is willing to spend thousands on a private car plate which is only for vanity reasons, where as with a domain name, it can be used to build a valuable business.
I personally think good short domain names (.co.uk / .uk) may become more valuable in the future. I know the google update killed off the values of long-tail keyword domains, but short, one word memorable domains (i.e. Money.co.uk should hopefully get more valuable).