I am certainly not saying that the judge did not know what he was doing. That does not mean he was right in deciding that Depp was a wife beater.
Libel verdicts are almost impossible to appeal unless the judge has made an error in law. The judge did not make any such error. Depp's lawyers tried to argue that the judgement was perverse, but the bar to make such an argument is very high. They had to show that no-one could possibly have arrived at that judgement based on the evidence. Even if they could show that every other judge would have come up with a different verdict, that would not have been enough. It was clearly open to the judge to come to the conclusions he did based on the evidence presented, so the attempt to appeal was rejected. That does not necessarily mean the original verdict was correct, nor does it mean that the judges in the Court of Appeal would have arrived at the same verdict if they had heard the case.
The judge largely accepted Heard's evidence, rejecting evidence from police officers, medical practitioners, one of her friends and others that disagreed with her version of events, and rejecting contemporaneous documentary evidence that disagreed with her. He found her a credible witness. However, we know that part of the reason he accepted her version of events was her statement, made under oath, that she had given the $7M divorce settlement to charity. It appears that statement was a lie. Would the judge still have accepted her evidence if he had known that? We don't know.
The other point I would make is that the judge decided the case on the balance of probabilities, the standard used in the UK for civil cases. Some people seem to want to take this judgement as proving Depp was a wife beater beyond reasonable doubt. That is not the case.
The judge in the UK case refused to make a third party disclosure order against Heard. Whilst that decision was correct, it meant she was able to limit her disclosure to evidence that favoured her. She did not have to disclose any evidence that undermined her version of events. In the US case, she will have been forced to disclose everything requested by Depp's legal team, regardless of whether it helps or hinders her case. This case may therefore include significant evidence that was not included in the UK case.
The judge in the UK case knew what he was doing. He is a very good judge - one of the best. But he is not infallible. He may have been mistaken. Juries and magistrates get it wrong in criminal cases, where the prosecution has to prove its case beyond reasonable doubt. Judges will, therefore, get it wrong in civil cases where the standard of proof required is much lower.