There is a specially reserved domain type for registered companies and, if you have a limited company (Ltd) you can be quite sure that the domain name yourcompany.ltd.uk is available. This domain type was specifically created to put right the damage caused by insufficient control in the early days of domain registrations.
The domain type .co.uk was intended for businesses but no checks were made on the nature of the people who were registering. If you register a .ltd.uk domain it must be made in the name of your company and the application will not be allowed until it has been checked against Companies House records. When you see a domain ending in .ltd.uk you can be sure that it is a legally registered company.
For public limited companies a similar option exists, with the same rules and controls. The option in this case is .plc.uk.