I got some clarification on station access from First GW, and thought I would share it.
Until the mid 1980s, British Rail had barriers on the platform where all tickets were manually checked. People needed a platform ticket to get through these checks, if they were seeing someone off or meeting a train. So when British Rail went over to the current system of 'open stations', the public could come and go as they pleased - and the need for platform tickets disappeared.
Some ticket offices did still issue platform tickets if they were asked, although they weren't legally required or even part of our ticket range. Collectors often bought them and we were getting more and more requests to post them out. This caused a lot of extra work for booking offices, when really their priority has to be the customers who need to make travel arrangements. So now we don't issue platform tickets at all.
Instead, our colleagues assess each non-traveller's reasons for being on the platform individually. They are trained to consider safety, security, the reason someone gives for wanting access, and so on. They use their professional judgement to make reasoned decisions.
Therefore, [regarding enthusiast access] I would suggest that upon arrival at one of our stations, you ask to speak to the Duty Station Manager for their authorization.