Slitherine

In the game, that is.

In the editor you can set an event to assign a skin to 'all' units of a particular nation.
This works fine for ie Germany and Italy.
But it simply does not work for Britain, Australia or India (those are the ones I checked).
The skin reverts back to editor_default_skin whenever you click on a different trigger/event.

You know this already but I will ask anyway. The skins that you are trying to use, are they custom? Because, if so, they would need to be in the default skins folder and listed in skins.txt for this to have a chance of working. I doubt that the developer would design this trigger to work with the custom skins folder. The trigger itself is probably programmed to work in conjunction with skins.txt and, even though it may initially be allowing you to look at the custom skins folder as well as default, it will not "hold" for that reason. Besides, if you are using custom skins and changing defaults, this would mean making your campaign / scenario into a mod.

It works on units which are skin compatible models. Tanks or guns does not matter. It works on those units which you can change manually - they are skin capable.

Let's hope they will remake all models to accept skins

These are all default/official skins.