IN general: You need to be sure xyz.com/something redirects to xyz.com.uk/something with a 301. on EVERY link you have. That's the way to successfully do a migration. 301s are the key. If you put the 2 domains under the same website record, and set up UK one as primary, then NS should properly emit 301s to the same path but in the other domain. Which is the most effective way not to lose traffic i believe. Don't use "redirect domain" since that redirects everything to 1 url and 1 url only.