Give it a rest
Come on, how many times have you run this same story now?
First, it seems pretty clear that there is a VERY strong public interest in this story, irrespective of whether or not there's an explicit public interest immunity clause in the law. It's blatantly obvious no prosecution would or could ever be brought. First, the Director of Public Prosecutions can decide to block ANY case if he decides it's not in the public interest to proceed. And secondly, no jury in the land would convict the BBC for this.
Complaining about the BBC giving money to the Russian Mafia, all of the $800 or whatever it was? How is that different from undercover reporters exposing credit card or identity card scams by paying fraudsters for false documents, from exposing corrupt officials in the police or tax office who will take money to look up someone's details, from paying someone to smuggle their reporter in so they can report from Zimbabwe or Sudan, or any one of countless other examples where they might be giving money to criminals but you probably appreciate the results. Come to that, how is it different from the police paying an informant to prevent a bigger crime?
I'm sure the journalists at the BBC (and elsewhere) have very strong guidelines on not doing anything to encourage people to break the law who wouldn't otherwise have done so, but clearly there are plenty of situations where paying a small amount to a petty criminal to expose a much bigger issue may be justified.