I'm not sure if you're asking a technical or philosophical question there.
The technical part:
Military Control is a diplomatic action. It has a % chance of success based on the amount of ICs in the initiator and the target. If the initiator has more IC, the bigger the difference, the greater the chance of success. If the initiator has less, the greater the difference, the less chance of success. Depending on the scenario, the US can have 200+ more than the UK. This drops the chance of success to near 0. Contrary to popular opinion "acceptall" does not guarantee success with every diplomatic action. It raises the odds to the point where most actions will be successful. But the odds are too low in this case. I haven't tried it, but I'm reasonably confidant you can do it by event. It's not hardcoded or you probably couldn't do it by editing the save game.
By the way, my instructions were for editing a saved game not an AI file. But if control of Finland is passed by event, you should be able to change the probabilities to whatever you want.
On a philosophical note:
I've seen a lot of games where the AI US laid down, wrecking an otherwise interesting game. If some UK player wants to take control to save a good game, I can totally sympathize.