Connection
There are two ways to use the whois.  You can access the whois through the Nominet website here or you can access it through port 43.

The format of the queries to port 43 is the domain name followed by carriage return and line feed:

<domain name><cr><lf>

So an example of a whois query is:

internet.co.uk

Limits
These are given in the acceptable use policy .

If either of these limits is exceeded then further queries are rejected with the blocked message.  The length of the block depends on how many queries have been made so far.  For example, if you send 1 query per second for 1,000 seconds then you will be blocked after that for approximately 23 hours, 43 minutes and 20 seconds.

Responses
All queries to the Nominet whois will return a response.

There are various types of responses:

Response: Where the domain name is already registered the response will use the following format:

    Domain name:
        <domain name>

    Registrant:
        <registrant>

    Trading as:
        <trading as>

    Registrant type:
        <registrant type>, (<number type>: <organization number>)

    Registrant's address:
        <registrant address>

    Registrar:
        <tag holder name> [Tag = <tag>]
        URL: <tag holder URL>

    Relevant dates:
        Registered on: <registered date>
        Renewal date:  <renewal date>
        Last updated:  <updated date>

    Registration status:
        <registration status>

    Name servers:
        <nameservers>

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

Notes

• The "Trading as" section will be omitted if there is no information to display here.
• In the "Registrant type" section everything from the comma onwards is only shown if the type is one that can have an associated number, such as a registered company number, registered charity number or registered school number. A list of all the registrant types is available.
  
• If there is no registrant type then this section will say "No registrant type listed.".
• If there are no name servers then this section will say "No name servers listed.".
• If the domain name is suspended then the "Registration status" section will include an additional line after the status, that says "***  This registration has been SUSPENDED. ***" 
• In some cases there is no registration agent and so the "Registrant's agent" section may display either of the following:

      No agent listed.  This domain is registered directly with Nominet.

or

      No agent listed.

• The"Registrant address" section may display the following if legal registrant is an individual and has opted out of the Nominet WHOIS.:

        The registrant is a non-trading individual who has opted to have their
        address omitted from the WHOIS service.

Response: This is the response for when the domain name passed to the server is syntactically correct, is a .uk domain, conforms to the name rules but is not found:

    No match for "<domain name>".

    This domain name has not been registered.

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

Response:  This is the response when a query is blocked on the WHOIS/WHOIS2 service because the originator of the query has exceeded its quota:

    Error for "<domain name>".

    The WHOIS query quota for <xxx.xxx.xxx.xxx> has been exceeded 
    and will be replenished in <nnn> seconds.

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

Response:  This is the response when a the domain name passed to the server is syntactically incorrect.

    Error for "<domain name>".

    <reason>

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

• The <reason> can be one of the following:

    One or more parts of the domain name exceeds the limit of 63 characters.

    Domain names may only comprise the characters A-Z, a-z, 0-9, hyphen (-)
    and dot (.).

    The domain name contains too few parts.

    The domain name exceeds the maximum length of 256 characters.

    One or more parts of the domain name were of zero length.

Response:  This is the response when the domain name passed to the server is syntactically correct, is a .uk domain, is not found in the registry but does not conform to the name rules.

    Error for "<domain name>".

    This domain cannot be registered because it contravenes the Nominet UK
    naming rules.  The reason is:
      <reason>

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

• The <reason> can be one of the following:

      invalid format for a .sch.uk domain name.

      the domain name contains too few parts.

      the domain name contains too many parts.

      third-level domains may not comprise one character.

      third-level domains may not comprise two alphabetic characters.

      third-level domains may neither start nor end with a hyphen.

      third-level domains may not start with "xn--".

Response:  This is the response when the domain name passed to the server is syntactically correct, and is either a non-.uk domain or is for a second-level .uk domain that is not in the registry (e.g. .police.uk).

    Error for "<domain name>".

    Nominet is not the registry for this domain name.

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

Response:  This is the response when the WHOIS server has a transient error accessing the register database.

    Error for "<domain name>".

    There was a problem accessing the database.  Please try again.

    WHOIS lookup made at <hh:mm:ss dd-mmm-yyyy>

--
<copyright text>

Response:  The copyright text will read as follows:

This WHOIS information is provided for free by Nominet UK the central registry 
for .uk domain names. This information and the .uk WHOIS are: 
    
    Copyright Nominet UK 1996 - <current year>.

You may not access the .uk WHOIS or use any data from it except as permitted 
By the terms of use available in full at http://www.nominet.org.uk/whois, which 
Includes restrictions on: (A) use of the data for advertising, or its 
repackaging, recompilation, redistribution or reuse (B) obscuring, removing 
or hiding any or all of this notice and  (C) exceeding query rate or volume 
limits. The data is provided on an 'as-is' basis and may lag behind the 
register. Access may be withdrawn or restricted at any time.