However the railway is private property so if you’re asked to leave you must do so. Refusal to leave when instructed means you’ll be technically committing railway trespass.
Staff don’t need cite a particular breach of the bylaws to ask someone to leave (albeit in the majority of cases where things escalate it’s likely that one or more bylaws will indeed have been breached). It could simply be due to someone being distracting, or acting in a way the staff member considers intimidating.
This is not correct as respects a ticket-holder.
Whilst the railway is private property, it is generally known and accepted that any member of the general public can enter upon stations without specific permission. This is referred to as "implied permission to enter" and can be withdrawn as respects any individual person, or people generally, at any time with or without a reason (although not, for example, for a reason that would constitute discrimination on a protected ground). An individual's permission might, for example, be withdrawn because they are begging for money, and people generally have their permission withdrawn when a station closes for the night.
Ticket-holders, however, do not use this implied permission to enter and remain on stations or trains. They have permission to be on the relevant station/train by virtue of the contract they have entered into; the railway has granted them this permission by selling them a ticket. The railway cannot unilaterally back out of the contract and require a ticket-holder to leave a station for no reason.
The railway byelaws provide that a passenger in breach of (most of the) byelaws can be removed from the station. A law can override the contract, and as such a ticket holder who does breach the byelaws can be removed. But the railway has no right in law to require the holder of a valid ticket to leave if they have conducted themselves legally. And filming on the railway is, in general, legal.
A non-ticket holder or a passenger who has reached their destination would not be within scope of the above and is susceptible to having their implied permission to be on the railway withdrawn.
You do not have an inalienable right to film staff with your mobile just because you don’t approve of body cams.
Actually, you do, since filming staff is legal.