[daminator]

1. Edit the skin files. Dont know what platform you are using but if it is Windows, it is located here... C:\Users\<username>\AppData\Roaming\Kodi\addons\skin.aeon.nox.silvo\16x9
Open the file Variables.xml and then search for UK:12A and you will see something like this...
<value condition="String.IsEqual(ListItem.MPAA,UK:12A) | String.IsEqual(ListItem.MPAA,United Kingdom:12A)">BBFC_12A_Certificate_UK.png</value>
Change the first UK:12A to your format. Nothing else on that line needs changing. Do that for each of your ratings, should be easy to spot as they are all grouped together. This occurs 4 times for each rating in the file.

It worked! :-)

Thanks Karellen. I just did your first method. Replaced all "UK:"s with "GB-"s. Restarted Kodi, and now it all works great.

Thanks again,
Damian