National Rail's guidance for rail enthusiasts (a.k.a. 'train spotters') states "You may require a platform ticket to allow access to platforms"
http://www.nationalrail.co.uk/stations_destinations/51952.aspx
so they're obviously still available in some places.
In practice though the staff at the ticket barriers are often quite flexible about letting people through to assist arriving passengers, even without a platform ticket. (Just ensure that you look for the same member of staff when you leave the platform, so that you don't get accused of having just travelled on a train without a ticket).