Public Suffix List
The Public Suffix List (PSL) is a catalog gathering eTLDs (effective top-level domain). The domains listed in the PSL are then considered as TLDs.
Sectorial domain names (such as "police.uk") are part of this list.
What are the implications?
The PSL is mainly used by browsers (Mozilla's, Chrome and Chromium or Opera) but is also consulted by certification authorities during the audit process.
It is forbidden to issue a wildcard certificate for an eTLD, the same way it would be impossible to obtain a SSL certificate for "*.com" for example.
"uk" is a TLD. In therory, it should be possible to register the "co.uk" domain. Actually "co.uk" is listed in the PSL and is therefore an eTLD. It is then possible to get a wildcard certificate for "*.example.uk", "*.example.co.uk" but not for "*.co.uk".
How to register an eTLD in the Public Suffix List?
Registrations are mainly managed by registrars. However, any registrant can request their domain registration in the PSL in order to better control its use.