For your model, not only the map has to be on the card but so does the application.
(this is assuming the US map folder is on the sd card)
Make an Explorer, not Home, backup of your unit's contents. Ensure hidden files/folders are showing. Now, DOUBLE CHECK that you do indeed have a backup on your computer.
Once done, delete everything from internal.
Then, with the card in the unit, connect to Home. It should offer an application. Accept it, install it. Disconnect the unit properly from Home. Use Home's Device Disconnect icon.
Shut down Home. Make sure it is not running in the background in the notification area.
Connect to the computer again without Home running and look at the contents of the card. Does it show a bunch more stuff than just the map folder? If so, disconnect from the computer properly and restart. You should have the voices available.
(As an aside, if all is working, you can restore your backup to the unit (not card). When you want the UK map, just start without the sd card in it.)