The way the game is now, it is considered cheating, but you can to some extent.
You can opening the console (all commands listed by entering '?<Enter>' and enter the command 'changefaction GBR<Enter>'. You will still have the interface of the faction you loaded at the start, but you will now have control over the GBR forces. So you can give them orders, but you cannot build units.
I only did this to prevent the British from doing some moves I found to be poor, and otherwise let Athena do her thing.
If you've found that Athena made a poor move, you can always go back a turn, and change those moves, using the above method, because when you go back a turn, all the orders issued before executing the turn, including those the AI issued are still present, but now you can see and change them.
If you use changefaction before turn execution, you will be responsible for issuing all moves of that faction, because there is only one '_.ord' files per faction, and when the AI encounters it, believes that a player is controlling that faction.
The next turn, when you click the 'Advance to the next turn' button, you may be asked if the AI should take over that faction again. Simply answer yes and continue.