Drew-
Sorry I don't know of any Janko keyboards in London or Belgium.
Kristine Naragon's 1977 book lists these Janko keyboards in Europe:
Gesellschaft der Musikfreunde in Vienna, Austria
Staatliches Intitut Fur Musikforschung, Preussischer Kulturbesitz
Musikinstrumenten-Museum, Berlin, Germany (This is just a keyboard
without a piano case)
Germanisches Nationalmuseum, Neupert Collection,Nurnberg, Germany
Kunsthistorisches Museum, Samlung Alter, Musikinstrumente, Vienna, Austria
They let me play the one in the Gemeentemuseum in the Hague, Netherlands
in 1988, but then they had never met anyone who knew how to play one
before. After I played a few pieces on the piano, the curator said he
wished I had let him know I was coming a few months in advance, so he
could have arranged a presentation/concert/lecture. That would have been
fun.
If you contact the curator at any of these museums, there is a good
chance they will let you play. It's worth a try.
Paul V.