Check if you have en_GB.SRD in /.System/ASR (for voice command to work)
Make sure that voice is set to British English Emily and that text is british english and keyboard is british english.
Hard Reset Nuvi and set to United Kingdom.
If you use Express to Download English it should get all the files (Device Optional Updates Language Files English)

Check inventory to see if your JCV file is listed. (File must be listed in Inventory) if not listed
Recheck your path to the JCV file make sure it is \Garmin\JCV\D3048120A.JCV and not \Garmin\JVC\D3048120A.JCV (common mistake)
Maybe your JCV file is corrupt. Make new download.