Another difference between the UK and the US is the way evidence is presented. In a civil case in the UK, a witness gives a written statement. In court, the direct examination (i.e. questioning by the side that called them) is limited to confirming their identity and that these are their witness statements. Their time in court is almost entirely taken up with cross examination. In the US, the direct examination leads the witness through their testimony. This can mean that we hear far more of the witness's version of events.
In this case, this has resulted in us getting Depp's full version of the alleged head butt. In the UK, we only got a partial version when it was put to him in cross examination. According to Depp, Heard was attacking him - hitting and scratching him. He managed to get her in a bear hug in an attempt to stop the attack. this was not entirely successful as she resorted to kicking him. She then stopped, claiming he had head butted her and broken her nose. He was adamant that any contact was accidental and that her nose would not have been broken. According to Depp, Heard then left the room, returning several minutes later with a tissue up her nose which she showed him, telling him he had made her nose bleed. She then threw the tissue in the bin. He says that he later retrieved the tissue and found that the red substance on it was nail varnish, not blood.
This is, of course, Depp's version of events. I am not saying it is correct.