As far as I'm aware there are actually two builds of the Nexus in the UK - ERE27 for those who imported from the USA, and EPF21B for those who bought from Vodafone. Seeing as it's an ERE27 build that's been available first, I would assume (please tell me if I'm wrong) that those who imported from the USA will get updates before other builds.
The update should just come as a notification, but you can manually check under Settings -> About Phone -> System updates
As usual, the OTA update is staggered. It seems that the press has been getting the update first (since they have some of the oldest N1s around), and over the course of the next few weeks, more and more users will receive the update.