When first announced back in July 2013 it was US only, but it is now coming to the UK, (and 10 other countries: Canada, Denmark, Finland, France, Germany, Italy, Netherlands, Norway, Spain, and Sweden).
On the Official Google Blog, Mario Queiroz (VP of Product Management, Chromecast) writes:
In addition to your favourite apps like YouTube, Google Play Movies, Google Play Music and Netflix (where available), we're working with local content providers to bring even more of the movies and TV shows you love to Chromecast. Apps will start rolling out today, and include BBC iPlayer in the U.K; France TV Pluzz and SFR TV in France with CANALPLAY coming soon; and Watchever in Germany with Maxdome coming soon. So instead of huddling around your laptop to watch Sherlock solve the next crime or getting caught up on all the workplace drama in Stramberg, you can cast it, sit back, and watch together on the big screen.
Chromecast will keep getting better. We recently opened up Chromecast to developers, and in a few short weeks more than 3,000 developers worldwide have signed up to bring their apps and websites to Chromecast. You’ll soon have more TV shows, movies, videos, sports, music and games to choose from. Stay up-to-date on the latest apps that work with Chromecast at chromecast.com/apps.
It's compatible with Wi-Fi enabled Android 2.3+ smartphones and tablets; iOS 6.0+ iPhones, iPads, and iPods; Chrome for Mac and Chrome for Windows; and Chromebook Pixel.
Costing £30, you can buy it from the likes of Amazon, Currys PC World and Google Play itself.
You can read more about the development in The Guardian - see Google Chromecast goes on sale in UK, bringing Android apps to digital TV