Background Investigation and Credit Check

Generally speaking, as with most other organizations, probably not. However, again as with other organizations, if your role is within accounting or finance, for example, yes it could perhaps be an issue. However, credit checks aren't to see how your credit rating is; the intent is to see if there are arrears due to collections or if you have accounts with collections, if you've declared bankruptcy, and if there are judgments / pending court cases.

My understanding is that a CCJ is failure to make a repayment, correct?

If so, yes this could potentially be an issue, more so if you haven't disclosed it already.

Always remember translate is the key with the BI. We all have secrets, their job is to find out and they will.

Tbh... This is exactly the judgements they look for civil cards, regardless of jurisdiction.

This is not a reason not to continue the process, and no need to mention something at this stage, as you have not been asked to do so; you go ahead with the interview as in business as usual. When the time comes to disclose any issues, you do it by being truthful and accurate. You will get at that point the chance to put your story in context. Until that point, no need to worry too much about it.