On the 14th of December 2005, I was very lucky to travel on board a GNER Eurostar from Peterborough to Waterloo International. Admittedly, this was not a public service, but the GNER Eurostar farewell trip, about a month after they stopped using them.
It was great fun standing in the buffet car, consuming refreshments, whilst riding along the North London Line. I recall being informed that we must take our passports with us, although, in the event, we didn't need to show them.
Some of those on board were a little disappointed that a white set was provided (3312+3311), rather than a GNER blue one, but it was a highly memorable trip, all the same.