Looks like audible.com is doing their alien original drama for free on the US site.
Follow the link http://www.audible.com/alien
If it redirects you to UK site just click the "Based on your location you have been directed to audible.co.uk. Do you want to go to audible.com ?" banner at the top of the screen to go back to US and you should see the offer, if not follow the link again and it should stick this time.
You should see a big banner for it on the 1st screen.
Follow through and you can buy it for $0.00
I logged in with my normal amazon account and it seems to have worked.