Here is what the Police and Criminal Evidence Act states:
3.3 The custody officer may authorise the interviewer not to audio record the interview when it is:
(a) not reasonably practicable because of equipment failure or the unavailability of a suitable interview room or recording equipment and the authorising officer considers, on reasonable grounds, that the interview should not be delayed;
or
(b) clear from the outset there will not be a prosecution.
Note: In these cases the interview should be recorded in writing in accordance with Code C, section 11. In all cases the custody officer shall record the specific reasons for not audio recording. See Note 3B
Note 3B states: A decision not to audio record an interview for any reason may be the subject of comment in court. The authorising officer should be prepared to justify that decision.
See Sec 3, Interviews to be audio recorded:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/117585/pace-code-e.pdf
(whether the same rules apply to TFL, I don't know).