Google has never been 100% clear on how to address this specific issue since serving up different versions of a site depending on where the user is located is considered grey hat, but obviously it makes sense from a user's perspective to provide the most relevant content to them as possible. As you stated, the site crawler/search engines will only access the version of the site accessible to them depending on the IP address they are using at the time so that explains why only the US version of the site was crawled. If you changed the IP to one in the UK, then it would likely crawl that version of the site.
Anyways, I think the best route would be to set up a new sitemap for each version of the site and submit them in Google Webmaster Tools. I came across this blog post which did exactly this and it seemed to resolve their problems - http://www.stateofsearch.com/how-to-use-multiple-sitemaps-on-one-domain-for-geo-targeting/. Hope that helps!
