.LTD.UK domains are tied to limited companies - so for example:
If your company name is "Example Company Ltd" you will only be permitted to register EXAMPLECOMPANY.LTD.UK
As the domain name must match the company name exactly you would not be eligible to register "EXAMPLE.LTD.UK" or "EXAMPLECO.LTD.UK".
Because of this restriction, Nominet (the UK domain authority) manually check each .LTD.UK domain name registration to ensure it meets their requirements and that the details provided (company name and number) are correct, it can therefore take a few days before the registration completes.
If your company name doesn't match the domain you apply for exactly they will reject the registration.