Your sim password is known as a PIN code. If you have forgottan it, you need to call your network provider to get a PUK code to unlock the PIN code.
Enter your PIN code incorrect 3 times and it will ask for a PUK code, this is when you enter the PUK code given by your network provider. After you have entered your PUK code, you can then set a new PIN code.
Some UK networks now allow you to view the PUK code online, OR by using an automate system. In order for this to work, you either need to setup an online account or know the automated system security questions.