The .co.uk top level domain is a generic domain which is used mostly by UK companies. Similiar to .com, there are very few restrictions on what names can be registered, and who can register them. A UK presence is NOT required to register in this TLD. Until recently, because .ie domain registrations were limited to the Republic of Ireland only, .co.uk was the correct domain for most companies located in Northern Ireland. Now it is common for NI companies to register in both .ie and .co.uk.

The .uk top-level domain is broken up into several subdomains including .org.uk for non-profit organisations , .ltd.uk for UK limited companies and .plc.uk for UK public companies but it is the unrestricted .co.uk which is by far the most popular. It is used both by UK companies (even those who would better suit the other classifications) and non-uk organisations who want to reserve their names or trademarks in the UK market.

Having a www.company.co.uk website is the most common location for company websites for UK organizations (including Northern Ireland).

Registering in .co.uk can help protect your company name and/or trademarks from being used by others in the UK market, even if you don't yet want to set up a website.

Registering a .co.uk domain is useful in giving the impression that a company has a UK presence.

The .co.uk top-level domain is not as frequently used as .com, so it may not be peoples first-guess for the location of your website unless you are a UK organisation.

Because there are so few rules for registering a .co.uk domain, and because of its popularity, there are millions of domains registered already. This makes is less likely that your first choice of name will be available.