According to the manual, there is a skirmish mode when you are in "solo".

Does this exist? And if so, where is it? All I can see in skirmish mode are the preset campaigns/ operations.

From memory, it is one of the buttons on the lower left. It is a little distracting that when you press Solo, the screen above changes to a listing of campaigns, but look back down again and you will see the buttons have changed.

OK, I see it now, thanks