You can download the Alison Keenan language pack for Asterisk from here.
Once you have replaces all default Asterisk prompts in /var/lib/asterisk/sounds directory with Alison Keenan’s sound files they will not work correctly out of the box. Use the following bash loop to re-sample the .wav files at a sample rate Asterisk will be happy with.
for file in $(ls *.wav); domv -f $file convertfile sox convertfile -r 8000 $file echo "Converted $file" done
Make sure you run this script in all sub folders of sounds.
One more thing, make sure you also copy over the beep sounds from the default sound pack as they are missing in the Alison Keenan British language pack.
Hope this helps.