Neither of the two units that went into the tunnel (319008 and 319009) are currently in service, but one of them is being converted to a "bimode" Class 769 and is earmarked for Transport for Wales.
The reason units from that rather humdrum class were chosen for the honour was that no Eurostar train had yet been completed and so it was, at the time, the only type that could operate on the electrical supply on the London to Folkestone route, and also the supply in the tunnel.