Based on Chris Titus’s gist, changed for GB.

echo "LC_ALL=en_GB.UTF-8" | sudo tee -a /etc/environment
echo "en_GB.UTF-8 UTF-8" | sudo tee -a /etc/locale.gen
echo "LANG=en_GB.UTF-8" | sudo tee -a /etc/locale.conf
sudo locale-gen en_GB.UTF-8