If you wish to cancel a domain name you can do so by logging into your Online Services account and selecting the Cancel Domains option. You should only request the cancellation of a .uk domain name if you no longer want it and this process should not be used to change your registrar or to transfer your domain name to another registrant.
Once a .uk domain name is cancelled it will be available immediately to be re-registered on a first-come, first-served basis by any applicant. Often domain names that are cancelled are re-registered very quickly and there is no guarantee that you will be able to re-register it before someone else.
Once scheduled for cancellation the domain name will be removed from the register on the day of cancellation at some point before midnight. At this point the domain name will be available for anyone to register.
You should only cancel a domain name if you are sure that you no longer need it.