This is your opportunity to be completely honest with them and it's important that, when you decide to come clean, that you are honest and deal with everything in one go.
Did you or did you not short fare as well in this time?
You say that you are worried they will find your ticket purchases suspicious.
Are you able to substantiate your journeys, with evidence, on multiple one-stop tickets wholly within London for which contactless is cheaper and easier? They will have detected this too.
London Bridge to Blackfriars is about 7-8 minutes cycling and is quicker than leaving the bike there and taking the train. The investigators weren't born yesterday and deal with this stuff all the time. Remember they have already got all the information they need by auditing your ticket purchases; you are now the subject of an honesty test to see if you've learned your lesson.
Oh right sorry I see what you mean. I also need to admit to not buying the full ticket from ND to LDN BF. In a stupid anxiety ridden panic after I got caught i deleted my trainline account so I can't see all the evidence but I have some emails, not all, but I have edited accordingly below:
Dear Fraud Investigation Team,
I am writing in response to the email I have received today to discuss my invalid rail ticket journey between North Dulwich and London Blackfriars on the 4th of February.
I want to sincerely apologise for failing to use the right railcard and wrong ticket when travelling between North Dulwich to LDN Bridge/Blackfriars on multiple occasions in a stupid attempt to save money on minimum wage. What I have done is incredibly foolish and inconsiderate to the train services and I feel deeply ashamed of myself.
Since being stopped at Blackfriars I have been researching the impact my actions have on GTR and I am truly appalled at my behaviour and how I have taken the railways for granted in such a way and have had sleepless nights thinking about how careless I have been.
I cannot express how much I apologise for my actions and since then I have been paying for the correct fares and using my correct railcard and I will continue to do so for the rest of my life.
I would be extremely grateful if you would consider the opportunity for me to pay a fine for my outstanding fares to resolve this out of court if this is possible, alongside any administrative costs to deal with this matter.
Once again I am extremely sorry for what I have done and I can guarantee you I have learnt my lesson and will not get on a train ever again without the correct ticket.
Yours Sincerely,