Eurostar can take you to Paris, or Lille/Brussels/Amsterdam. Availability for Paris, Lille and Brussels is usually better than those to Amsterdam.
From Amsterdam and Brussels and Paris there is a night train to Vienna, although places could be fully booked. Especially for the departure out of Brussels.
Should the Eurostar be fully booked, the best alternatives are bus (Flixbus, most of these take the ferry Dover-Calais) or as a foot passenger on the ferry from Harwich to Hoek van Holland. Latter can be done overnight. From there you have good acces to all points of interest in the Benelux.
As you want scenic rides, I can recommend these that are more or less on your route to Italy and included in the pass:
the Rhine gorge railway in Germany (trains via Koblenz, to Mannheim)
Zentralbahn in Switzerland (Luzern to Interlaken)
Rhätische bahn (Chur to Tirano)
The Semmerringbahn (Vienna to Villach/Graz)