I want to do King's Cross to Peterborough travelling out around 16:00 and back later in the evening. National Rail gives me the £24 fare which I assume is a FCC only day return - I know East Coast isn't an option at that time without breaking the bank. However, I would like the option of returning on East Coast which the FCC only won't allow. Can I get a ticket which will allow me to do that? I see there is a £28 off-peak by travelling East Coast earlier which I assume is an 'Any Permitted'. Would I be able to buy one of those or similar and use it on a FCC outward during the East Coast peak and so have the option of returning on a fast train?