I just managed to get an order through, with some help from the info in the CAG thread. Started trying around a bit over an hour ago. Opened a few browsers and kept refreshing both links with the .co.uk and the .com domains to add to my chances, just make sure you know which site you log into once you manage to do so, having an account on one creates it for both but logging in only logs you in for that domain. Once I managed to login, used this link to automatically add the game to my cart:
http://www.gamersgate.com/shop/addtocart?sku=DDB-BETHESDANYC
Then I got an email from gamersgate saying the order went through with an order status address in it:
https://www.gamersgate.com/shop/orderdetails/###
where ### is your order number.
Then you can go straight to:
https://www.gamersgate.com/shop/payment/###
using that order number to get to the page with the link to the pay with paypal button, it should redirect you to paypal and you can pay. Once that happens ( you may have to refresh as paypal automatically tries to redirect you to gamersgate, I left it at the loading page after I hit confirm for a while, then hit refresh and it took me to a page where it told me I had already paid for this product or whatever and gave me a return to merchant link which I then had to refresh a few more times until I got to the gamersgate confirmation page with my receipt.
Hopefully that helps some others get through if you are committed enough, im yet to get through to my games page to access the product codes but since I have confirmation from both paypal and gamersgate it should be fine.
Edit: Just got through to my library, all the games are there. Trying to get one of the keys to double check now.