So companies like Serco-Abellio should be the arbiters of the law? This fortunately does not feature in any civilized society I know of.
The very thought fills me with horror.
No, but if a member of staff follows company procedure in good faith, the
company (or those responsible for defining the procedures), not the member of staff, should be prosecuted if those procedures are found to be deficient.
For example, if Merseyrail's policy is that once the doors are closing they are not reopened unless obstructed, and it is found that this is what happened and she walked into the closing doors and was knocked off balance by them, then there is an argument that that procedure may be deficient. But the guard is doing as he was told, and may have been under threat of disciplinary proceedings if he ignored the policy, and caused delay by reopening, for example.
FWIW, this is potentially an odd edge case where the hustle alarms (which are a retrofit job on 50x) make things less safe, as you now push close to start something happening in about 10 seconds (by which time you may well be standing too far away from the train on a curved platform to stop it in time), not to start it now. It's also a case where on a curved platform (don't know if this was or not) DOO might have been safer, as a DOO driver wouldn't have to stand away from the train on a curved platform to see all doors closing safely, but could instead have his finger resting on "open" while watching the doors close on the monitors (or, as an alternative and as happens on passenger controls for doors on some UIC stock, you have to keep your finger on close until the doors are fully closed and interlock gained, otherwise they reopen automatically as soon as it is removed). It might be that guard-operated trains are seen to need a door remote control to allow that to be afforded to a guard.
(I don't know if this is what happened, it is hypothetical)