Oh, yeah, I forgot that important detail! It’ll be in a bike bag. Pretty sure mine is an old version of the Planet X bag.
The Eurostar bikes page says “Bike Over 85cm (in bike bag): Registered luggage service: On day of travel: £25/€29 each way”.
The Eurostar luggage page says “If you’re travelling with extra, oversized or restricted luggage, you’ll need to use our registered luggage service, provided by EuroDespatch…We’ll send your luggage on the first available train, as long as there’s space. No matter what, we guarantee that it’ll be ready to collect at your Eurostar destination station within 24 hours of registering it.” It says “Luggage, suitcases, bags, etc (maximum weight 30kg per item) £20 each (up to 3 items, £40 for each additional item)”, but also has a row for bicycles which links back to the previous page, implying that bicycles should be in bags and also saying “to make sure that your bike travels on the same train as you during that time, you’ll need to book a space for it with our registered luggage service”, which is completely at odds with what the restricted luggage service page says.
Don’t know about anyone else, but I’m none the wiser.