Ok, so we will just give you the fares for your connections as of now. As result you can take a look, sum it up and make a decision.
London-Paris: for August 3 there are actually tickets available for as low as 65 USD (which is great price for this connection). Those are for 4 pm train. For the morning trains fares are from 121 USD to 148 USD (or 73-89 GBP). Tickets can be bought on
EuroStar websiteParis-Rome: on the 5th of August fares for direct overnight train is rather big (from 120 EUR), while for daytime options with change total fare would be even bigger, thus you might consider here flights, which are cheaper (from 60 EUR) and will get you to Rome faster.
Rome-Naples: here you have plenty of options. You might go with purchase of discounted ticket for high-speed ES trains (36-41 EUR depending departure and 1:10) or InterCity (from 18 EUR and app. 2 hours). You can check this fares on your own from TrenItalia website (will need offer called "Mini"). Or don't bother with purchasing anything in advance, but just use regional train, fare is always the same (only 10.5 EUR and 2.5 hours), so you can buy your ticket even 5 minutes before departure.
For Napoli-Pompei -
http://www.vesuviana.it/web/en/OrariNaples-Florence: for morning August 10 there are still discounted ticket for 50 EUR available for high-speed trains (app. 3 hours travel time). For InterCity trains (app. 5 hours) - from 40 EUR.
Florence-Pisa-Florence: everything easy here, because of multiple regional trains for only 5.8 EUR one way. So just come to the station before departure and get your ticket with no problem.
Florence-Venice: also discounted tickets for InterCity or ES trains available. For instance, for August 12 afternoon from 26 EUR on ES. While regular full fare for ES is 43 EUR, for InterCity - 24 EUR.
Venice-London: here it would definitely make sense to fly, because of expensive rail travel. For instance, for 14th evening there are RyanAir flight for 68 EUR to London Stansted etc.