How do you know what your puk code is?

You get it off your network provider.

Adam
January 2010

Telecommunications service is regulated in the UK by Ofcom. On 25 July 2003, Ofcom introduced the General Conditions of Entitlement which apply to all communications networks and service providers in the UK.

Condition 18 requires all providers to provide number portability but only to subscribers of publicly available telephone services who request it. Number portability must be provided as soon as practicable and on reasonable terms to subscribers and bilateral porting arrangements between providers must accord with agreed processes.

1. Your current Service Provider must give you a Porting Authorisation Code (PAC) or a reason why it cannot be issued within 2 working days of your request. You can obtain your PAC by calling the customer service group of your current network. Usually your network operator will be able to provide your PAC over the phone and will then follow this up in writing.
2. Your PAC is valid for a period of 30 calendar days and your request must be submitted to your new Service Provider within that time.
3. Your number can then be transferred anytime between 2 and 32 calendar days from the date your PAC is issued.

If the customer is on a fixed term contract, they will often be expected to pay any outstanding sums owed to the end of the contract before they can port their number. Some mobile phone companies can charge a fee to move your number. This is usually no more than £25. The provider must issue you a PAC within 2 working days of your request even if you have not paid your bill.

Even more from Wikipedia
January 2010

Porting Authorisation Code (PAC) is a unique identifier used by some mobile and fixed-line telephone network providers to facilitate mobile number portability (MNP). This allows users to retain their mobile telephone number when switching operators.

Peccavi
January 2010