Each product page on Loaded clearly shows the region or country where a game can be activated. It is important to check this information before you buy, so you can be sure the key will work with your account.
Here is a quick guide:
Worldwide (WW): Can be activated anywhere in the world, unless stated otherwise.
EU: Can only be activated in Europe (including the UK).
UK: Can only be activated in the United Kingdom.
US: Can only be activated in the United States.
Other region restrictions will also be listed on the product page when relevant. Always make sure the region of the game matches both your location and the account you plan to use, or the key may not work.