Sorry to dredge up this topic again, but after looking at earlier postings, I still have at least two questions. It appears that a rail ticket and voucher for the specified attraction are the requirements. Is a one way ticket sufficient? It appears that a travel card purchased at a rail station (as opposed to tube station) which is valid on the date visiting an attraction is sufficient. Thus, a one day travel card would do? Is there a limit on the number of attractions that can be visited on that day? With a one day ticket, could one visit multiple attractions? On the national rail site, I find travel tickets mentioned as one day and 7 day, nothing in between. Also, no pricing.
A one way rail ticket is sufficient if it was valid on the day you use the discount. Do not feed the one way ticket through a barrier reader, it will be "eaten", go through a manned gate to keep the ticket. Yes, a one day travelcard issued by a rail station is sufficient. There is no limit to the number of discount offers you use in a day, you can visit as many attractions with a one day ticket as you have time for. One day and 7 day are the shortest term travelcards available. The 3 day travelcard was eliminated more than a year ago.
A brief addition to the above for those continuing a rail journey from London from the Days Out FAQs, "If for example you are buying single tickets, e.g.Advance tickets to London for a one-way journey on a Monday, and Advance tickets for departing London on a Friday you will be able to use the vouchers during the Monday-Friday period, provided you show both outward and return tickets (remember to tell the train company staff that you need to keep your outward tickets!)." http://www.daysoutguide.co.uk/faq