At the moment, for all they know, you could have passed the ticket to someone else who didn't buy a ticket.
Showing with CCTV that you bought a ticket doesn't prove you retained the ticket.
There may be a fee for retrieving the CCTV, although I understand this is capped at a relatively low level. It just might show what you did with the ticket. There may be CCTV on the concourse, platforms and trains.
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