It may have come as a surprise to many who read this article in The Times that unlawfully obtained material (e.g. via hacking) can be used in civil litigation. There is no rule of law that requires any evidence to be excluded from litigation because it has been acquired illegally, with judges tending to adopt a more "inclusive" rather than “exclusive” approach.
Ultimately whether or not to allow such material into evidence is a matter for the judge's discretion. As it was put by Master Davison in the case of Mustard v Flower (2019 EWHC 2623 (QB)) – a personal injury claim which allowed into evidence a covert recording of the claimant's meeting with the defendant’s medical expert, it is an exercise of balancing "the means used to obtain the evidence together with its relevance and probative value and the effect that admitting or not admitting it would have on the fairness of the litigation process and the trial”.
Whilst a party's legal advisors must never advise or instruct that material is obtained illegally (after all, it is a crime), it is likely that evidence that has become available as a consequence of such activity, and is plainly relevant to the issues in dispute, is likely to be permitted. It is even more likely where the material obtained would have fallen within the disclosure obligations of the hacked party in any event (Ras Al Khaimah Investment Authority v Azima  EWCA Civ 349). Whilst challenges may be made to the admissibility of material obtained illegally if there has been a breach of confidence or misuse of personal data, such challenges are unlikely to succeed on public policy grounds if the withholding of that material would conceal a crime (Imerman v Tchenguiz  EWCA Civ 908).
That the success (or failure) of a piece of litigation turns on the availability (and quality of) relevant evidence is not revelatory. However, given this, and the fact that evidence is now stored on computers (making it susceptible to all forms of hacking activity), we are likely to see more and more examples of cases where litigating parties take steps to obtain evidence through illegal means.