First of all, looking at your itinerary it seems that rail pass would be a wrong way to go here.
It is hard to operate with exact financial arguments here, since you didn't provide info about pass type (Youth, Adult, Saver etc.?), but to give you an idea - lets count:
1) Milan - Florence: most expensive regular tariff ticket for the high-speed trains costs 52 euro (if purchased in advance - much cheaper), regular ticket tariff for IC train is 27.5 euro, Regional trains <20euro
2) Florence -Monterosso: by regional trains - only 9.9 euro
3) Monterosso - Como: 14.55 euro (regional trains) and 32euro for IC/EC trains.
4) Como - Verona: from 12 to 28 euro (depending on train type)
5) Verona - Venice: 6.15 euro by regional train
6) Venice - Zurich: discounted international tickets Italy-Switzerland can be bought from 19 euro (
http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=e6eaf0bde7aca110VgnVCM1000003f16f90aRCRD#4). Full regular fare 89 euro.
7) Zurich - Paris: regular tariff is around 100 euro, but route can be done cheaper
8)Paris - London: pass is not a solution - supplement is extremely high.
So, even we count the most expensive options (those bought before departure at the ticket counter) without Paris-London, then the full price for the journey will be app. 300 euro. If you go cheap, then you can manage for the sum twice as low. Plus you will have to pay certain seat reservations with pass as well, for instance for high-speed trains in Italy, Italy-Swiss and Swiss-France trains.
Also any comments about 1st v 2nd class - is it worth it??
Really depends on your preferences and thickness of your wallet