Arrive in London-1week. Make way to Croatia (have 4-5days to do so)
Hmm... do you really want to travel by ground all the way from London to Croatia? Because it will be much more expensive, then to fly, only London-Paris/Brussels leg by
EuroStar train probably will cost more than flight from London to Croatia (WizzAir, EasyJet etc.). Maybe it will be a good thing to go with mid solution - fly from London to somewhere closer to Croatia (Budapest, Venice, Triest etc.) and then just spend those days on getting from there to your Croatian destination?
Croatia-Italy: obviously ferries. Available from most of the major coastal cities (Zadar, Split, Dubrovnik etc.). Check -
www.jadrolinija.hr,
http://www.blueline-ferries.com/ etc.
Italy: withing Italy pass makes sense only for multiple long distance journeys with expensive high-speed trains. If you plan to do short legs like Ancona-Bologna, Bologna-Firenze, Firenze-Pisa etc., then pass is not worth it, since tickets for regional trains will be much cheaper, roughly saying from 5 to 20 EUR for such relatively short trips.
Italy-France: there is a special discounted tickets available for direct trains from Italy to France (trenitalia.com), discount is called 'SMART', you can purchase such tickets online from TrenItalia website. Better to do it in advance, since the number of those is limited.
France: pretty much the same story as with Italian domestic trips. Long distance journeys and usage of high-speed TGV trains tend to be expensive, but you can always use regional TER trains, especially since your plan is to "make your way up to Paris", thus you hardly will travel long-distance on every journey. You can check out fare here -
www.tgv-europe.com Resume: considering the info you gave us, the pass option probably isn't the best solution for you. Since in most cases, as far as we understand, you plan rather short trips where actual point to point fare would be cheaper, than the cost of the travel day of your pass. Loss of time with regional vs. high-speed trains is not that bad either, on short distance legs it isn't that essential, imho. Plus on some routes there is simply no high-speed trains available.
p.s. maybe if you can give us exact 'must' destinations for you, we can advice more precise advice