You may register an account on the jabber.org.uk service using this form if you are unable to do so from within your own Jabber client. You may register an account at the domains xmpp.org.uk and jabber.org.uk as well as setting up an email address incase you forget your password.
You do not have to enter in an email address, but if you enter it you will be able to reset your password and have it sent to you. If you do not have an email address set you will not be able to access your account if you have forgotten your password. Jabber.org.uk does not publish these addresses and they will only ever be used for this task.