Thanks for such a clear write-up, Rachel. As you say, there’s an interesting question about what exactly to do if the billing address and IP lookup disagree. I watched some videos of an HMRC seminar on this (these, I believe: http://www.online-web-presentations.com/HMRC/) and got the impression that the authorities will be quite flexible about what further evidence they accept as proof of location. So asking the customer as you suggest above (with a checkbox or similar) may well be okay.