In a criminal case it is the CPS that charge you, not the victim. Clearly if the victim decides they no longer wish to be a witness or to participate in the case it hugely weakens that case.
No, of course you cant bribe them but as I understand it, were they to launch a civil claim you could settle that out of order straight away and then 'anticipate' that the victim withdraws their involvement from the criminal case. The problem being that settling the civil case implies guilt....
Its only illegal if you get caught
I am guessing on all of this, by the way.