This restriction comes from Xbox Live capabilities, meaning those that don't live in a country supported by the service at launch won't be able to authenticate their games. The console requires you to sign in at least once every 24 hours.
Most of our readers will probably be safe as the UK is one of the 21 regions that will be supported, but it's interesting to see that the entirety of Africa has been excluded, as have Japan and China. This same restriction applies to the majority of countries West of the UK, with the exception of Russia. A handful of European countries will be supported, but a large chunk won't. Croatia, Turkey, Greece and even Portugal are amongst those that have seemingly been left out.
This information came from a pre-order information page on the Xbox website. A disclaimer on the page reads "Xbox One games are for activation and distribution only in specified geographic regions," a disclaimer on the Xbox One reservation page says."
Japan - which is excluded for the time being - is expecting an Xbox One launch in 2014, so we're unsure as to how long it will take for other countries to gain support.
There's a full list of supported countries below, as well as a map showing the ratio of supported to excluded regions (thanks, NeoGAF)