Thank you for contacting Amazon.co.uk
Please accept my apologies for any inconvenience with your pre order for "Defiance".
Please see the updated pre order information page for this title:
http://www.amazon.co.uk/gp/feature.h...d_i=B00B4QD3YO
As explained on this page, the Beta Event for this title has been delayed by the publisher. I am sure you will appreciate Amazon have no control over this.
Once the revised launch date has been announced by the publisher we will then send out your Beta Code.
Going by the information on the link you have posted, as you have ordered the X Box 360 version, the launch date is currently set be 25th - 26th March. However, as we have not yet been supplied confirmation of this information directly by the publisher, I cannot confirm this yet. Provided this date is not changed by the publisher you will have your Beta Code by launch date.
As control over these codes are under the authority of the publisher, at the moment I am unable to confirm exactly when your code will be sent.
Did I solve your problem?