Are the tickets print-at-home with a QR code on it, or e-tickets on your phone? If so, show them to the bod at the barrier or scan the QR code if there is a reader below the Oyster reader on the barrier. All platforms at Kings Cross are barriered and only left open occasionally.
If you have a reservation with a long number (something like 8 digits) that is not the ticket and you need to collect it from a machine or window using the same card you used to buy it and that number. Wait for all the tickets to come out of the machine - there will be lots. You need them all. Then put the ticket portions (will say FROM and TO on the upper left corner) in the gate or show them to the bod.
Then when the Train Manager or Ticket Inspector comes through the train they will want to see them again.
No validation in the UK though.....