From my and found experience org.uk's are hard sells, near impossible to companies or large professionals. They also assume rarities in normal search results. However, they do seem to sell a lot more now then the used to - not that I think this will amount to anything, especially with the new TLD's coming up which could significantly effect uses of the 'older' variants. (although these are more location|company geered, such as .wales or .amazon.
.co.uk and .com are very hard to place like namealot said. Generally I'd value .co.uk's as being about the same on a search scale - if not a bit less, due to the later introduction, but only for the UK. .co.uk's in general will seem cheaper because the UK tends to have less native searchs then US for obvious populations reasons.
.com's are great as they can enter both markets.
Cars.co.uk might be purchased for 1 million if it had 50,000 Monthly Unique UK searchs (find this out by setting local on google keyword tool), but the dotcom for 10 million if the US searchs added another 300,000. Then you have other countries using english which the .com can access easier. Figures are not actual.
In your example if we place relative sums, the .co.uk might be 100, whereas the dotcom 400 relative to the .co.uk's 100. A .org.uk might be up to 30. (but only really expect this is the name is generic or of non-company value).
Also, the issue of .org.uk's is debated. I personally see little in them but a lot of domain traders do - so their are buyers out there.
I'm not a expect in .org.uk's so take that with a pinch of salt.
