For the past 6 years I have traveled with my bicycle by Eurostar and SNCF to various parts of France to do the Etape du Tour. This year I booked the TGV to Annecy and then set about booking the Eurostar tickets. Quite by chance I came across a 'tweet' indicating that the Eurostar policy on packed bicycles had changed and I saw this confirmed when I searched the Eurostar site. I had a long conversation with Eurostar None of their suggestions were any use to me. I could not take the bicycle unpacked as it needed to be packed for the TGV. I could not turn up with it packed as there was no guarantee it would appear in Paris for up to 24 hours after I did, when I had a train connection in Paris. The only thing I might have done is sent it ahead but that would have required a special trip from Berkshire to St Pancras ahead of travel and a special trip back to St Pancras to collect it after my return. Extra charges were of course involved for all these options. In frustration and against every instinct in my fibre I gave up and booked a flight from Heathrow to Paris (packed bike conveyed at no extra cost). I did look at the possibility of train/ferry but ruled it out on grounds of time and cost.
It is interesting that when I first used Eurostar they used to cheerfully place my packed bike in the baggage carriage. More recently it appeared to be too much trouble for some train guards to open the baggage carriage and more recently still I had been told to dump it in the buffet car. I believe there are two baggage cars on each train which no doubt go at least half empty.
Throughout France on very similar trains there is no problem turning up and taking a boxed bike at no additional charge or hassle. Eurostar are not (quite) alone. I had planned to take my bicycle this year to Venice for the Grand Fondo Sportful. However in response to my enquiries I was informed curtly that the sleeper from Paris to Venice does not take bicycles. Again I had to pass my custom rather reluctantly to British Airways.