www.ena.org for starters. Then after that you might need to call them and see what they have for sites in the UK. I seem to remember manyyears back a paper version of the CEN test being given at one of the US Military bases. But that was so long ago that it makes my brain hurt to think about.
As I always say here, go to the source first, and then come here for discusion on what you found.
If you can find enough people there who are interested, maybe you can convince them to offer the test there.
Are you located in the US? If so you need a US RN license, and can find test centers by visiting http://www.goamp.com
Are you located in the UK? Your licensure is equivalent to a US RN? If so, you need to contact the BCEN for information on an alternate test site. In the US the test is computer-based - for international candidates
"Candidates living outside of the U.S. must contact BCEN to
determine what testing options are available. Alternate testing
requires a 90-day advanced scheduling notice."
Are you US Military living over seas? If so, talk to your DANTES coordinator - not only might they be able to help you set up the test, but they just might pay for it.