As much as I love long distance train rides, if you're dragging your bike along, rather avoid the trains. If they're anything like the German ones, it's going to be a huge pain in the ass dragging the bike through the (often crowded) stations for changeovers. Some trains also don't really have decent storage space, so the box will be in the way. You could DHL the bike from the UK to France, but it'll be pricey. If you book in advance the flights will be cheaper and faster. Another option: There are always a ton of Brits in France around that time. Maybe look on Pinkbike to see if you can catch a lift with someone.