lots of lovely places mentioned, so won't repeat them [mostly]. I'd maybe add Utrecht and Aachen, happy memories of both, and Aachen is not far from cities like Cologne and Bonn although I've not been to Bonn, if you're going on on the train, once you get to Cologne you can get to places like Berlin which is well worth visiting, or almost anywhere else in Germany.
As anyone mentioned Brussels, places like Ghent and Bruges are not far away, from Lille, where the Eurostar stops you can get to Douai which is a little gem, or it was the last time I was there.
To think that London, or anywhere else for that matter, does not belong to any one demographic