This answer will explain how to Lock/Unlock your 123-reg domain name. This answer only applies to .com, .net, .org, .info, .name, .biz and .la domain names.
To lock/unlock a top level domain name, you will need to do the following:
In the Domain names section, select the relevant domain name from the drop down box and click on the Manage button.
Click on the Manage Domain Locking option to lock/unlock the domain name.