To register .ltd.uk domain names there are special rules set out by Nominet and so these registrations are always manually processed by Nominet.
http://www.nominet.org.uk/uk-domain-names/about-domain-names/uk-domain-subdomains/second-level-domains
.ltd.uk Rules state "The name shall correspond exactly to the registered name save for characters not in the "allowable" set which shall be interpreted as below and the "reserved" words. Where the definite article appears as the first word of the name it may be omitted".
This means that you can register "MyLimitedCompanyName.ltd.uk" but most likely not "mlcn.ltd.uk" as this does not reflect EXACTLY your Ltd company name.
If you submit a registration for a .ltd.uk domain name please do submit a ticket advising us of your EXACT limited company name and company registration number OR let us know if we should modify the .ltd.uk domain name to match your Limited Company name and resubmit your registration to Nominet.
If you have already ordered a .ltd.uk domain name then your domain name will show as "Cancelled" until we submit the additional information to Nominet and have your registration successfully complete.