I think a lot of the people in the UK have been put off by the problems that some of the US users have been having. After reading this topic, I was going to leave it until it became downloadable on marketplace. But then I had some time to kill at work today and decided to give them a call. It all seemed pretty easy - I don't think the guy was too sure to begin with, but he put me on hold for a minute of two and then seemed to be aware of the problem. He only took the serial number of the box and not the console ID. After confirming the registration details, I was given the case reference and was told I'd get a call - I guess you need to make sure you register the box before.
I've used Microsoft support a lot in the past mainly for server/networking issues and they have been extremely professional - I have to say though, the support staff I spoke today did not seem anywhere near as professional. That said, they were still polite, friendly and courteous and certainly better than most support lines I've contacted in the past - just not as good as the main Microsoft support team.
To anyone who is in the UK who has been putting it off, I would say just go for it. It only took about 10 minutes of my time (mostly on hold waiting for a representative), and certainly wasn't as stressful as I thought it was going to be. I'll drop a note when I get my code.