I can see nowhere in the article that the information was stolen from a bank by the informant?
I do not think anyone should ever gain from an illegal act and if this is the case (again it does not say this) I think the UK government is wrong.
Liechtenstein is a tax haven. I guess as they are not EU members the UK and German governments are restricted on what action they could take.
Economic sanctions would be preferable to individuals profiting from illegal acts - if this is what has happened.
If the UK government has paid an informant for information he has obtained legally then I have no objection to this if it helps convict tax cheats.