With the updates
Legal part aside.
You know he cheated - and need to decide what it means for you.
You know he might have parented another child - and you need to decide how you feel not only about that, but also about his current reaction to this issue.
Some women would consider "take responsibility" first approach - want to see him to step up and be the father the other child deserves. Some would consider "influence of their own children" first - and prefer him to deny it as much as possible and not divide his resources.
This is your choice and it might make sense to tell him your position.
Legal aspect - the world is getting smaller, EU and UK are connected in many ways.
We don't know the child's parentage legal status in Czechia. Given the child is around 6-7, it is quite possible that her husband used the standard (extendable) deadline in Czech law and proven he isn't a father, that would explain "why now".
If that husband was released from being the legal father, he can be replaced by agreement of mother and the new father or by court decision based on DNA tests.
Your husband can cooperate (test & declare parentage) or let it go to court, drag his feet and delay things, but if he is the father and the court proves it, he will be charged with costs of the court and child maintanance backdated through the court time.
Of course we don't know the situation for sure, if her husband is still the legal father, it differs.
However, the legal aspect is more important for him now that for you, Opera. You are deciding whether he is good enough person for you.