I have been unable to find the GB English language pack for Magento 1.9.1 and have searched high and low, but it seems it has been removed since August 2014 (ref: http://www.magentocommerce.com/translations). I checked magento connect but nothing there either. I am hoping someone can confirm if I am doing the right thing to translate my store from US english to GB, I am relatively new to Magento but happy to work with system files, and I have done this: I have set the default local to English GB in my back end (system-configuration-general-general-locale). I copied the contents of the folder app/locale/en_US to a new folder, app/locale/en_GB (which did not exist in my fresh install of Magento 1.9.1). Having found some of the necessary changes (cart=basket etc) I've amended it in there and it is working fine on the front end. The instructions on adding a language pack manually to Magento are far more complex, but does what I have done work for en_GB at least for such a common language? I assume since en_GB is listed in magento then all I need to do is create the necessary translation files in the app/loale/en_GB folder. Any help or advise on the correct way or an alternative resource for the language pack will help!
You can also create in your theme folder a locale folder where you create a translate.csv. At the backend you can go to system->configuratation->general->design and set the theme folder after translation (on storeview). It is one file with all your translation lines.
Had the same issue whilst trying to locate the GB language pack. I am aware that just posting links as an answer is NOT the stack overflow way but this is where the language pack can be found --
ALL credit to Lightnest - www.lightnest.co.uk
Yep, en_GB has been removed. Clearly the American developers are determined to force us Brits to accept the terms "Cart", "Zip" and "Shipping" instead of "Basket", "Postcode" and "Delivery". Unfortunately those terms don't apply here or just sound plain wrong, so now every British developer has to spend hours making their own translation files, when some of the most obscure countries in the world have language packs readily available on Magento Connect. Fortunately I have kept the old British translation file from an older installation, it's somewhat incomplete but it's a start. When I have the time I will create a complete pack and make it available via my website.