Amtrak gift certificates can only be redeemed at staffed stations by a ticket agent or by travel agents.
On-line reservations, which are paid at the time of booking, cannot use gift certificates as payment. Phone booking can, but only when payment is being made at the station.
The interesting point would be the travel agent option. While USA agents can accept certificates as payment, I do not know about UK agents. The gift certificate payment and subsequent ticket issuance by a travel agent is a manual, hand-written transaction that reconciles through the Airline Reporting Corporation (ARC). I do not know if overseas agents process Amtrak tickets that way, or if the transactions are third party. If they also process directly through the ARC, maybe. However, my suspicion, knowing Amtrak, is that overseas agents will not be able to process gift certificates. That is just a guess.
