Ah, they've moved since they started. Their new offices are indeed in London, though if you look at the copyright notice on the footer of that page you'll see it has a Netherlands address. Their careers page [1] does say their new offices are in London.

When they started, the two founders were both still mainly post-docs at the Netherlands Cancer Institute [2], and that's what I remembered, reinforced by the copyright notice I mentioned above.

[1] http://www.papersapp.com/careers/

[2] https://web.archive.org/web/20070213053257/http://mekentosj....