no, don't EVER do it.
It's THEIR responsibility to decide when to overtake, IF to overtake. It's not your responsibility to BE overtaken.
You don't know how much room they need.
That's the moral position. Legally, though, HOWEVER, if you did do it and an accident happened, I personally still think you'd have a good chance of it still being their fault because it is still their responsibility to make sure it's safe and not to rely on your hand gesture, besides, your hand gesture could easily have been meant to mean something else etc, time difference between hand gesture and manoevre, etc.