As mentioned there is no one rule governing this. As a rule of thumb if there are no barriers onto the platform just walk on and start spotting and so long as you act responsibly there should not be a problem, if for any reason there is a problem a member of railway staff will come and have a word.
If there are barriers then just ask one of the staff at the barriers. Some require you to sign in at the station managers office where you will be given a ticket or slip of paper as a pass, at other stations some still issue platform tickets at the ticket office at a nominal price.