Sorry, according to the Planet Cruise website (which seeks to list all cruises and then lets you apply filters to narrow down your choices) it's not possible:
https://www.planetcruise.com
The only cruise which takes in both destinations requires you to fly back from Venice. It's a 24-night cruise with Princess Cruises, departing from Dover on 6 October, calling at Le Havre (for Paris), La Coruna, Cadiz, Gibraltar, Sete, Portofino, Livorno (for Florence & Pisa), Monte Carlo, Naples, Messina, Valletta, Kotor, Split, Ravenna and Venice. Unsurprisingly, it's not cheap!
https://www.planetcruise.com/en/princess-cruises/pacific-princess/06-october-2018/129913
The only way that you could use that cruise without flying would be to take the train back from Venice. It takes about 13 or 14 hours to get from Venice to London (and you'd then need to get back to Dover, unless you'd used a train to get to Dover when you set out), with a change at Torino and a cross-Paris transfer.
So it looks like you'll need to re-think your plans but I hope that the Planet Cruise website will still help you anyway. (Obviously, you don't need to book with Planet Cruise; you might find the same cruise offered cheaper elsewhere. Nevertheless, Planet Cruise is a good place to start your searches)