Can i ask, why are there excess fare windows in the platforms at kings cross if not for these situations.
They are there to facilitate the payment of fares before a passenger without a ticket reaches the exit barriers. They are also there to sell tickets arising from a wide range of other situations. I'm sorry that I can't quite see the significance of this question, so I may be missing your point, but I cannot see how it is relevant to your incidentof travelling without a valid ticket and after you had not attempted to buy another ticket (or an excess) from that window.
should I press the fact that he in essence stole my personal information?
I see no evidence of theft in your report.