You can try the next variant!
In text editor like Notepad for Windows you may open the file Garmin\Locales.xml in Nuvi. Make backup copy before you open it!!!
Then find in this file the next block.
Code:
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And change parameter <Units> to:
<Units>Statute</Units>

Further you need to reset device and again select "locale" for UK. Height must be in feet.
Attention!!! When you will reset device you will loose all data: waypoints, routes and setting. You will need make backup of waypoints, routes and restore its after reset.