On this route there are more options to get to Paris and from there to London with Eurostar. For wich trains are you looking for?

In general for Italian long distance and high speed trains you can book almost all trains via Austrian Railways online (Freccia 10€ and IC 3€) using tickets.oebb.at - one way ticket - discount code: Interrail/Eurail Global Pass. Or at ticket offices in Italy, Austria, Germany, Switzerland or Belgium. 

If you run via Switzerland you can reserve the Eurocity EC trains Milan - Geneva (13€ 1st class) most times via Austrian Railways in the same way like italian trains. But some times some trains are not available also when seats are still free. Or at ticket offices in Italy, Austria, Switzerland or Belgium. 

From Geneva to Paris Gare de Lyon online only via Eurail reservation service. Or at ticket offices in Switzerland and France (maybe Belgium). 

Or you go via Milano Porta Garibaldi with TGV to Paris GDL (only 2-3 times per day) reservation online only via Eurail. You could also use Frecciarossa high speed train from Milano Centrale to Paris GDL, but this is not yet part of Eurail Pass. Tickets are cheap throw. 

Paris GDL - Paris Nord you have to take the metro or RER trains or taxi (not included in the pass)

Paris Nord to London online available via Eurail reservation service or with a cover number of a paper pass via B-Europe. You get a pass cover number via customer service (but they are recently very busy). Or directly from the Eurostar terminals. By calling the Eurostar booking center: +44 1233 617 575 (outside the UK) or 03432 186 186 (within the UK), SBB Stations in Switzerland, Railtourguide (UK based travel agent), SNCF stations in France and SNCB international counter in Belgium. 

Thank you so much for your quick reply, Angelo. I had a lot of trouble running into issues with either timing or tickets not being available when I tried going the route of booking them via the carriers, so I went ahead and submitted a request with the HelpDesk.

These are the trains I’m looking for are as follows:

• 06:18 from Venezia S. Lucia on FR 9702 to Milano P.Garibaldi
• 09:10 from Milano Centrale on EC 314 to Zuerich Hb
• 13:34 from Zuerich Hb on TGV 9222 to Paris Gare De Lyon
• 19:13 from Paris Nord on ES 9055 to London St Pancras Intl (United Kingdom)

Since you know so much about Eurail, would you be able to book the seat reservations for me by chance, since every internet browser puts me in a never-ending “loading results state” whenever I try to do it on the Eurail page?

This connection is not possible. Milano Porta Garibaldi and Milano Centrale are two different stations. Not possible to make a connection in 23 minutes transfer time.

Even the other options aren’t a good choice if you love your money. There are trains with much lower supplements (if you care about this). But you need to change your itinerary anyway...

That connection is mentioned on the trip result that I got directly from Eurail when using the Rail Planner app. It says that you transfer to that station via public transport (which Eurail estimated at 15 mins). If I were to catch a cab out of the station, wouldn’t it be faster and therefore available as an option?

It’s possible to do in 3 minutes by underground. But I’d never rely on this if you aren’t familiar with the situation. And you’ll loose all the reservations up to London. Why taking such a risk? 

This route is risky. You could change your route using the

• Eurocity to Munich from Venice and than change on the nighttrain to Paris in Munich
• or taking the nighttrain in Venice to Munich and from Munich take ICE and TGV to Paris (with 1 change) and from Paris to London with Eurostar. 
• or taking the nighttrain in Venice to Munich and from Munich take ICE to Brussel (with 1 change) and from Brussel to London with Eurostar. 
• or taking Eurocity Venice to Innsbruck change on the nighttrain to Amsterdam and take from Amsterdam the Eurostar to London. Or you change in Cologne to an ICE to Brussel and from Brussel an Eurostar to London. 

ICE has only optional reservation (except France) and you can book easy via bahn.com (only seat option).

EC to Munich or Innsbruck has also optional reservation, but you have to pay a fee. You can pay it at italian stations (and you get a free reservation on top) or on board of the train. You can book it also online (reservation extra 3€) using the same steps of booking italian trains via Austrian railways. 

Nighttrain Nightjet of Austrian Railways works for the reservation also like for italian trains, but be aware that many trains in summer are sold out. 

Eurostar like I describe in the first answer. 

My recommendation for a day trip would be this:

08.18 Venezia Santa Lucia to Milano Porta Garibaldi (10.47)

12.10 Milano Porta Garibaldi to Paris Gare de Lyon (19.12)

21.13 Paris Nord to London St. Pancras (22.44)

Stress-free, but high supplement for the train to Paris.

Or:

07.40 Venezia Santa Lucia to Venezia Mestre (7.50)

08.00 Venezia Mestre to Milano Centrale (10.15)

11.20 Milano Centrale to Basel SBB (15.42)

16.34 Basel SBB to Paris Gare de Lyon (19.41) - possible to get this much cheaper with one more change of trains at Mulhouse, departing Basel SBB at 16.21

21.13 Paris Nord to London St. Pancras (22.44)

Forget all the daytime options if the date on your app screen is true (22 may) - there are no passholder seats left for the eurostar to London after 11.13 - so none of the daytime solutions are possible - even not your original one.