Ok, here's what you do.
First, sign up a Eurogamer account here.
Then, go to the Call of Duty 4 beta give-away page here
. Type in your gamertag and click the confirm button. It'll give you an activation code.
Now go to The Call of Duty 4 UK Beta
sign-up site. Type in the activation code from Eurogamer, you'll get a confirmation email and you'll be entered for the first wave of of UK beta codes on the 10th of September.