I heard this tally ho in a youtube video (British). I also heard it in the movies "Jack the Giant Slayer", and "Spiderman 1". I understand it's some kind of expression or exclamation or idiom to say in certain circumstances.
I've googled for this, I just found that it is used to say a huntsman's cry to the hounds on sighting a fox, and a cry of tally ho. First, I don't get it, and second I can't find a connection between that "tally ho" and the videos I heard it from.