The Man in Seat 61 is regarded as possibly the most authoritative source of information available about continental rail travel:
https://www.seat61.com/Italy.htm
The best source of timetable information though is actually the website of the German railways. It brings the timetables of all European rail operators together into a single resource:
https://www.bahn.com/en/view/index.shtml
Using that site helps you quickly find the best routes into Italy. (e.g. I just asked it for journeys tomorrow from St Pancras International to Florence. The shortest journey time is 12h 35m, changing in Paris and Turin).
Note that, while Eurostar runs to Gare du Nord in Paris, trains to Turin use Gare de Lyon. You need to take RER Line D (towards Malesherbes/Melon) for a couple of stops to make the change. You need to buy a ticket at the machines in Gare du Nord or, if you want to save time you could buy an all-day travel ticket for central Paris at St Pancras International (in the Eurostar departure lounge). That would cost you more (at around a tenner) but it would save you having to bother about about queuing at Gare du Nord.
If you're starting from somewhere other than London, remember that you can select any major National Rail station as your starting point on the Eurostar website. That often gets you into London far cheaper than buying separate National Rail and Eurostar tickets. (It doesn't include the cross-London transfer, if required, though. Simply use a contactless debit card to pay for your Tube journey).
Details of Interrail passes can be found here:
https://www.interrail.eu/en/interrail-passes/one-country-pass/italy
However rail travel in Italy is fairly cheap anyway. Search here for times and fares:
http://www.trenitalia.com/tcom-en
(NB: If you buy individual tickets in Italy, they're not valid for travel until you've inserted them into the little machine at the platform entrance. If you forget to do that a ticket inspector will assume that you're a fare dodger!)
While the German railways website is the best one for sorting out international timetables, it's the French railways website which is the best one for booking (non-Eurostar) international travel:
https://uk.voyages-sncf.com/en/?visitor_country=UK#/
Lastly, don't expect a train in Italy to run on time!