Buying Advance Train Tickets in England

I have been on the NationalRail website and located the Midland Mainline train that we (there are 3 of us traveling) need to take out of London Saint Pancras. There is a big difference in prices (7# to 56#). 7 pounds if I buy the tickets in advance, and the higher for flexible fares. National Rail doesn’t sell the ticket and refer me to the Trainline. The problem is that they are not set up to except a postal code outside the UK. (We are in the US)

A railpass would not be cost effective as we only need to make one roundtrip ticket to Derbyshire for the weekend.

Someone said to use FastTicket – not sure how that works and if I can book & pay for the Advance tickets.

Does anyone know how I can order and receive the tickets in the US ?