Various users have reported battery drain problems with there Galaxy S3. While the phone is on standby, cell standy uses around 20% battery power.
Today Samsung has released an update for the Galaxy S3 unbranded UK version (BTU). The 1MB update will update your modem to version XXLH1 and fix the battery drain issue. The update is available using the normal channels (KIES or OTA).
The update is currently only available for BTU versions of the phone, if you also have the battery drain issue, it is possible to convert your phone to the BTU version so you are able to install the update.
Change your phone CSC to install the update
If you are not sure if you have the unbranded UK (BTU) version, or you want to convert your phone to the BTU version, there are various ways to do it:
1) Install the csc app from the google play store, this will allow you to check if you have the BTU Galaxy S3, and if not will allow you to change it. Note that this will wipe and reset your phone, so do a backup first!
2) You can also change the CSC code of your phone to BTU by typing *#272*imei#* on your phone. (to get your imei# type #060#. Note that this will wipe and reset your phone, so do a backup first!