That is poor. Not good at all. BTP know full well that the TOC has every right to deny you access to the CCTV if you claim you weren't there, as if you genuinely weren't, they would be in breach of the DPA.
If you know you are innocent, I would go back to the TOC and state that you can prove your whereabouts at the time in question, and that you believe that the fare evader has given false details using your stolen ID. State that you have tried to involve BTP (if you took the name of the person from BTP who spoke to you, even better), but for them to be able to investigate further, they would require the statement from the RPI and CCTV evidence.
My guess is they will back down. Hopefully they will agree to provide BTP with any information, if so, you then get back on the horn to BTP and tell them that Mr/Ms X at the TOC would be more than willing to cooperate in providing them with CCTV. Hopefully then this sorry episode can be put to bed.
And my apologies for my earlier post, but as you must guess, the Shaggy defence (it wasn't me) is one that has been heard by ITAL/TOC prosecution departments (and pretty much every law enforcement people) many, many times before.