There were quite a few reasons that they wern't ideal for use with GNER. The slow doors was one thing, which was particularly a problem on the semi-fast Leeds services they operated. Another problem was the length, which meant that the doors in the end coaches (IIRC it was the front 2 and the back 2) had to be cut out. The pantographs, being set up for European OHLE, had a habit of bringing the wires down. The fact that both pantographs had to be raised didn't help either. IIRC, they were restricted to 110mph north of Grantham, due to issues with OHLE, power supply or both. Running with both pantographs up also caused problems at neutral sections, since the APC cut power before the first pantograph entered, and didn't re-apply until the rear had exited. This cause quite a loss in speed, particularly at the N/S near the top of Stoke Bank, and the one north of Retford for trains stopping there, since the N/S is just before the junction were the Down platform loop re-joins the through line. Finally, the need for ECS moves to and from North Pole each night, or in the event of problems during the day, was far from ideal.
This isn't saying that the use of Eurostars wasn't a great idea, providing much needed extra capacity, but it would never have worked long term.
. LO was specially converted to accomodate Eurostar sets, and they even put a bloody big sign on the outside saying "L'Eurostar s'habite ici" or somthing (The Eurostar lives here!) I think there were a couple of test runs between the south and Man Picc but nothing ever happend. Last time i saw the inside of the Eurostar shed was in Rail mag I think, and it was being used to accomodate class 101's- back when they were slogging away on the Marple / Rose Hill circuit 
