If you haven't made your reservations for the Eurostar between London and mainland Europe, I recommend you to do that ASAP. 

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. On popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

If you don't get any result, press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

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)