In Great Britain, special train ticket destination exists for London International (CIV) and for London Eurostar (CIV). It seems prices and flexibility sometimes differ. As far as passenger rights are concerned, is there any difference between the two destinations, assuming I wish to connect to a continental Eurostar (not Dutch Flyer) destination?
Note that this answer is purely from personal experience. I've never managed to find any official reference.
My understanding is that tickets involving London Eurostar are only sold by Eurostar themselves in combination with a Eurostar ticket, whereas tickets involving London International may be sold independently.
Either of these tickets require you to show a valid international ticket (Eurostar, or ferry ticket for a London International fare) for the same day (or the day before/after in some cases, allowing an overnight stay in London). In practice, I've never been asked to show my Eurostar ticket at a station gate or on board a train, only when buying the ticket.
As far as I know it isn't currently possible to buy a ticket involving London International online (it has been in the past and may be in the future), or from any automated ticket machine. You can buy them from a ticket window, you'll need to show your Eurostar or ferry ticket.
My experience in asking for a ticket to London International at a ticket window has been all over the place. Sometimes I got it straight away, sometimes the employee spent some time checking their fare manual or consulting with colleagues. I've twice ended up with a ticket from London Eurostar — my understanding is that the employee wasn't supposed to issue this ticket, but I didn't argue with a cheaper fare against someone who was supposed to know better than me. Once I didn't manage to convince the employee that this fare existed and gave up because my train was about to depart.
The prices and restrictions are based on commercial considerations. Depending on your destination, there may be different fares and not all combinations exist.
In terms of CIV protection, there is no difference: you're guaranteed to be rebooked at no expense if a prior leg of your trip was delayed. It's not even clear to me that a CIV ticket is at all necessary — the CIV rules don't explicitly require that the tickets for all the legs of the journey are bought at the same time. But a CIV ticket reduces the need for arguing, and is often (but not always) cheaper.