If you're thinking about registering a domain name, the first thing you'll need to do is check whether your first choice of name already exists. One way to do this is to put it into a browser and to see if that returns anything. If it doesn't, that doesn't necessarily mean that the domain isn't already owned by someone.
Equally, you may want to know more about a domain that you already know exists. Maybe you want to find out when your own domain name expires, or when a certain domain name was registered. Maybe you just want to be nosey!
To do any of these things, you need to perform a "whois". Some operating systems have this facility inbuilt. However, it's very simple to get this information online too.
.uk domains
The best place to do a whois on any domain that ends in .uk (.co.uk, .org.uk etc) is Nominet. They are the provider of all .uk names - any domain name provider offering these will ultimately be registering them with Nominet (albeit sometimes via a 3rd party). You can access Nominet's whois service here:
Just type in any .uk domain and you'll get more details. If it returns "no match" then that domain name is available for purchase. Otherwise, you will get details about the organisation the domain is registered with and the registration and expiry dates of the domain. It used to be the case that address details of the organisation would also be available here, but this was removed when GDPR regulations were introduced in 2018.
Other domains
There's multiple websites that provide a more general whois service for all domains (including .com). We'll just include one of these here:
Again, just type in the domain in question to get more information. This site specifically tells you if a domain is available to buy (although we wouldn't necessarily recommend the service they link you to if you did want to buy it). If the domain is owned, you will get expiration and registrar details. Again, you will see "Redacted for Privacy" against the ownership details for the domain for GDPR reasons.