Here's a London station to which you've almost certainly never bought a ticket: London International (CIV). National Rail tickets to this station do exist, though they're rather hard to come by, and only available under very specific conditions.
The astute reader will have noticed the first issue: London International isn't a station. Of course, London does have an international station — St Pancras, from where the Eurostar to Paris and beyond departs — but it doesn't go by the name of 'London International'.
(There is of course, Stratford International, which has absolutely no international trains whatsoever, but that only adds to the confusion here, so we're going to ignore it... as Eurostar continues to do).
In this case, London International does refer to St Pancras, and these tickets are available to anyone travelling by train from other UK stations to St Pancras in order to take the Eurostar.
How (and why) to buy a ticket to London International
So why buy one of these tickets? If you're heading from somewhere beyond TfL's reaches to catch the Eurostar, why not just buy a normal National Rail ticket to St Pancras?
Tickets to 'London International (CIV)' offer a few benefits that regular train tickets don't, the most important one being that if your UK train into London is delayed (a rarity, we know...) and you miss your Eurostar as a result, Eurostar will put you onto the next available service where there's space. Additionally, if you arrive into London at a station that isn't St Pancras (or King's Cross), tube travel from that station to St Pancras is included with a 'London International (CIV)' ticket.
Before you go trotting off to your local station to buy one of these special tickets for posterity (let's face it, paper tickets won't be around forever), know this: You must have a Eurostar ticket in order to buy one. For this reason, they're not available to buy online or at ticket machines, as you'll need to show proof that you are indeed heading over to the continent — which makes us wonder, would these tickets have met their demise if the widespread closure of ticket offices had gone ahead?
Now, hands up, we've never actually bought a London International (CIV) ticket ourselves (though here's proof that they do exist), as we didn't even know about them the last time we took a jolly to Paree. People online have reported that sometimes, even ticket office staff aren't aware of them — and have provided the handy hint that LNE is the three letter station code for these tickets.
'London International (CIV)' tickets are available from most — though not all — UK stations. To check whether it's available from yours, head to the BR Fares website, and enter your station in the 'Origin' box, and 'LNE' in the 'Destination' box. Once you hit 'Query fares', if if comes up with any sort of 'Euro' fare, you're good to go — though, as we say, you can't buy the ticket itself online.
So what's 'London International (CIV)'?
One final question though: if LNE is the station code, what's CIV when it's at home? It refers to the International Convention for the transportation of Passengers (their weird use of caps, not ours), a set of conditions relating to cross-border travel (more on which here). Which is basically a formal way of mentioning that deal in which Eurostar will fit you on the next available service should you miss your train from St Pancras due to domestic UK train delays.
The National Rail website has more info about CIV tickets, and the unimpeachable Man in Seat 61 has, as always, a wealth of information on the topic. Almost worth booking a night or two in Paris just to get your hands on one of these rail rarities, don't you think?