I'm guessing it was before your change at Paddington, or after you got on at Kings Cross? as there is no direct service from reading to potters bar and would have to travel through london?
This looks like a clear cut Regulation of Railways Act 1889, Section 5 offence. I'm guessing that you gave him an admission of intending to not pay for your ticket.
I would start saving now. at the least you are looking at £100 - £200 + the cost of the fare if they offer to settle out of court, If not and you are convicted, the fine could be much higher, and the conviction would show on a DBS (formally CRB) check
TOC's are generally swift at processing offences, but can take up to 2 months before your letter drops on your doorstep. This will be a letter of intended prosecution. Basically they are giving you the opportunity to further have your say, before deciding what course of action they will take.
At this point i would probably grovel. Apologise again for your actions, that this has scared you into always purchasing a ticket in the future and if there was anything he could do to rectify things, You have pretty much admitted it and apologised during your interview with the inspector.
One would ask though how you got through London and the other part of your journey?