Whenever I've seen someone off there I've just asked to be let through and never had any issues. There may well be such a thing as a platform ticket but I've never needed one.
Some stations do not require you to purchase a platform ticket if you explain to the staff what you intended to do. They may require you to sign in instead.
Where platform tickets are sold, they cost 10p and are valid for 60 minutes.
It's not a compulsory ticket area, so you don't need a platform ticket.