Private prosecutions like this are specific to England & Wales (Scotland has a separate legal system where I believe they are extremely rare; Northern Ireland is also a separate legal system).

While the incentives are not to reveal evidence the law is absolutely clear that all evidence identified must be revealed. The difficulty is that forcing and unlimited discovery to identify the evidence would break the whole legal system, and this is what parties can hide behind (for a while at least). We'll have to see when and how anybody will face the consequences of any failures to reveal the evidence.

I very much recommend reading the Judgements in Bates vs Post Office. As litigation progressed you can notice how it becomes apparent to the judge how Post Office legal tactics and behaviour undermined the prosecutions.