I already have an Off Peak Return from Lomdon to Folkestone which is not valid on high speed services and the price was £34.90. If I wanted to upgrade for use on the Highspeed route from St Pancras for the outward journey only how much would the excess fare be? I don't want to do the return journey by the highspeed route as I want to stop off in South London on my return journey and it will be more convenient to come back to Charing Cross.
I don't normally buy walk on fares in advance but purchased them at the same time as I got my Advance Tickets from Nottingham to London for the other half of my journey where I wanted to travel 1st Class.