You can buy a single ticket or a carnet of 10 tickets from the buffet.
In 2013 there was a markup of around 10-20% and they only had carnets available.
As of September 2015, the markup was around 50% (£15 @ 1.37EUR/pound, compared to €14.10 in Paris). They were also selling single tickets at about a 20% markup (£1.60 each – €1.80 in Paris).
They sometimes mention the availability of the tickets in the PA announcements on the train.
Sources: my own experience (March/April 2011, September 2015), discussion in the comments, this webpage
I do recall Paris metro tickets being available on the Eurostar and at the information desk in London, but that was a few years ago and may well no longer be true. The price is rather more expensive than if you get them in Paris.
There generally isn’t that much of a queue in Paris Nord, especially outside peak hour. This is one of Paris’s busiest stations and one Eurostar isn’t that much of a load. Most travelers head for the most visible ticket selling machines, but there are many machines in different corners of the station. Head to the far end of the platform (where the suburban services leave; suburban ticket selling machines, such as the one pictured on Seat 61, sell metro tickets as well). Or head downstairs into the metro/RER area, and spread out a little; that’s often enough not to have to queue much if at all.
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