:?
There is no .com.uk, I assume you mean .co.uk?
.uk generally takes registrations at the second level, the big three open extensions are:
.co.uk for companies
.org.uk for non-profit organisations
.me.uk for individuals
.net.uk is supposed to be for ISPs (it is very rarely used), .gov.uk for government agencies, .ac.uk and .sch.uk (educational organisations), .ltd.uk, plc.uk (registered Ltd and Plc companies) and then a few others for the health service (.nhs.uk), police (.police.uk) and armed forces (.mod.uk)
.co.uk is by far and away the leader in number of active sites and potential resale value, like in most countries with long-established extensions, UK users tend to think of .co.uk before .com.
.org.uk is used in the UK, but can lose traffic to .org and .me.uk has not been around long enough to really establish itself.
If you were referring to .uk.com then this isn't a UK extension, but subdomains run by CentralNIC (who also offer .uk.net, .gb.com and .gb.net). I personally don't like these (reg fee too high, extension potentially too confusing for users and danger of CentralNic taking .uk.com away as what happened with .web.com)