.com are domains for international commercial entities. However, because the web got much of its start in the US, many of the .coms were taken by US companies (they do actually have .co.us). Also, due to the fact that the 'dotcom' name is well recognised, often they aren't used by commercial entities at all, for example personal home pages.
.co.uk domains are for commercial companies based in the UK.
i dont think that they do have .co.us. they do have .us though. we are the only country that i have seen that add the .co before the country prefix.
ie .nz .au .fr etc
There is nothing to stop a company using either .com or .co.uk, some countries such as France don't all people from other countries to register domains with their prefix without a lot of hassle and others let anybody do it.