I've stopped taking my bike on Eurostar.
The bike boxes supplied are too small for a bike with a rack and mudguards, even a quill stem is too much for the thin cases... They are more like a flight case for a bass guitar than a touring bike.
On a fully assembled bike it's the bikes clashing together on the hooks that will damage the frame.
Especially the paint work. So protect with cardboard.
Especially if you have just forked out on an Argos respray
French porteures will happily pass you your bike from the train rather than wait at eurodispatch.
Shame really because it could be so much better, for no extra cost to the company.