Like some (but not all) other country code domains, the .uk country code is subdivided, with registrations allowed only under second-level subdomains, not directly under .uk (though there are a handful of "grandfathered" names, like parliament.uk, registered directly under .uk before the current rules went into effect).
Nominet runs the largest-scale .uk domains:
Managed by others, not Nominet:
A lot of people sloppily refer to .co.uk as a "top level domain"; it isn't. It's a second-level domain beneath .uk.
There's also a similar degree of domain abuse to that in .com, where everybody and his stepmother, commercial or not, ignorantly gets a .co.uk domain for their site (if choosing a U.K. country-specific address) where in many cases .org.uk or other 2nd-level names would be more appropriate.