You might not need to go nuclear just yet - I'd call their customer services team again and ask to speak with someone senior (e.g. a supervisor/manager) - don't be rude, but also don't take no for an answer.
Once you have someone senior on the line ask again for the prosecutions team's contact number. Expect the answer to be the same (I suspect they don't take inbound calls just because of the number of cases they have on the go and the fact that any callers would tend not to be the most polite to them), when they don't give it to you ask for them to get the prosecutions team to call you. Get them to commit to a callback within a specified timeframe - e.g. before 3pm.
(As an aside, if I were in your position I would want all communications with the prosecutions team to be by letter/email so that there's a paper trail.)
If this is unsuccessful, write a full and complete letter/email to the CEO (
Charles Horton) explaining exactly what happened and how you have been treated so far. Explain that your intention has always been to pay the fare from Penge West to London Bridge and that at no point have you been given the opportunity to do so.
Ask for a response to your letter/email within five working days, and resolution within 14 days. Explain that your next step will be to escalate the matter to transport focus and/or the DfT.