In my experience of several stations and access to platforms.
1) Ticket machines do not know what a platform ticket is.
2) Some booking office staff do not know what a platform ticket is but they can usually muddle through.
3) If there are no staff at the barrier then the barriers MUST be open anyway.
4) I have seldom had barrier staff stop me anyway and then they usually only direct me to the ticket office for a platform ticket.
5) Platform tickets are only valid for an hour and do not work in the barriers anyway (so you have to show the paper ticket to barrier staff). I have never been told off for going over one hour or re-using a ticket.
My biggest problem with loitering on platforms is the Oyster prepay. You get fined (£4.40 iirc) if you enter a station through the barriers and then exit the same station - even if it takes 15 seconds. If your journey takes too long you get penalised and the daily cap stops working. So I have thoroughly explored the access to platforms without a ticket scenario in London. Biggest problem is remembering if I have been enjoying the view from the platform to go back out and THEN touch in if I am then actually travelling somewhere. Oyster is very vindictive - George Orwell was right.
One thing to bear in mind is if you are hanging around at the end of platforms you could be viewed with concern by railway staff. To save them the time and effort it is courtesy to tell them what you are doing anyway.
So for those of you interested in railways
have a look at this link. It tells you what you can/cannot do and what you should expect from the railways and vice versa.
