Police officers need access to their bodycam footage in certain situations in order to accurately file investigations. Obviously they would seek to be taking the correct notes at the time, as that's exactly what they relied on prior to the use of bodycams, and the contents of their notepad can then be submitted in evidence for court if needed. If they didn't file the paperwork precisely correctly in accordance with what were on the bodycam footage however, it would completely undermine the prosecution if they had some minor variance which, though not significant to the charge in itself, were enough to sow the seed of doubt about the officer telling the whole truth and nothing but the truth.
As noted above, any abuse of bodycam footage such as looking at it for resons other than a police investigation, or sharing it with someone not involved, or incorrect protocol for request, storage and disclosure of CCTV more generally, is a very serious form of misconduct and has seen officers lose their jobs. It is treated as seriously or even more seriously than a doctor looking up someone's medical records who isn't a patient of theirs, it's very serious malpractice.
If you're captured by a bodycam on the railway you're likely to be entitled to request the footage under the GDPR too, which is as it should be. Whenever you walk past some random person's door filming you they may be keeping that footage of you with absolutely no way for you to know who they are.