As a member of rail staff it does amuse me that we have the best part of four pages of opinion from the assembled "experts" on railway legality, all of which ignores a number of very basic fundamentals.
The first of these fundamentals is the meaning of a prohibition notice. Someone has already touched on it but would appear to have been laughed out of the thread, but if you pass any sign with a picture of a person in a red circle with a red diagonal line through them you have gone past the point where you should be. Irrespective of the wording below the pictogram informing passengers that they must not cross the line except by means of the footbridge/underpass, this sign is a marker delineating the public part of the platform from the private. In fact, within the Rule Book this sign denotes to staff that you must not pass beyond this point unless you are carrying out emergency protection of the line. Therefore not even staff are permitted to pass such a sign in order to seek shelter under a signal even if, like me, they hold a current and valid PTS certificate.
The second fundamental is the responsibility of rail staff for the safety of everyone on the railway. I can't speak for other drivers, but I count the far ends of platforms, especially where a person is standing beyond the prohibition notice or on the wrong side of a barrier, as the "suicide spot". If I see someone standing there I will consider calling it in to the signaller, even if only for his/her information. I couldn't care less if someone is or isn't committing a criminal act or trespassing, if I consider them to be "at risk" then I will call it in. I've already run over one person so far in my career and she was standing in a perfectly legal position on a platform before rushing out and jumping in front of my train. What happens after I've called in a person "at risk" is outside of my area of expertise, but I would imagine that such a report would result in some sort of action, even if only on the basis that the signaller and controller would not want to ignore a report in case the person in question flings themselves under a train or carries out some other nefarious activity.
The only thing that does appear to be a bit heavy-handed, especially if we are to take the OP's report on face value, is the issuing of a ticket and a fine. If the OP really had passed a prohibition notice or gone around a barrier to take shelter under a signal in all ignorance as he suggests, then I would have hoped that the worst the BTP would have done is to tell him to get back where he should be. However, we don't know for certain what if anything the OP did to aggravate the BTP. After all, why should we believe the OP rather than question the actions of the officers? Both positions are equally defensible, especially as we've only had the OP's side of the story.
That aside, there are too many unknowns in this story to make a firm judgement on whether the OP did anything wrong or not, and none of us are really qualified to make any judgements on the legalities of the situation.
O L Leigh