We did Dover to Rome but it was with Crystal, not Celebrity. We stopped in Lisbon, did a night passing of Gilbralter (beautiful at night), a port in Spain (can't remember!!), Portofino (Italy), Nice, Cannes and finally Cita-something. I can say, but can't spell it. You'll dock there when you go to Rome... it's about an hour by bus to Rome.
Sounds like Lisbon might be the same for both trips. We didn't do any excursions there. I spent 5 years in southern Spain as a teenager and we vacationed in Lisbon often. It's a beautiful old city as long as you get into the historical sections. It's also a dirty old city... especially where we docked. YUCK. I would suggest that you go see the castles in the area... walk the plaza/shopping/waterfront downtown. OR, get out of town completely if there's a tour that does that and see some of the countryside. It's gorgeous. The fishing villages are nice, too. Lancers and Mateaus wines are cheap (and good). Pottery (painted) is a local craft.
About the itinerary... the English Channel was one of the roughest rides on a cruise ship I've ever had. If you are prone to seasickness be prepared. I never get sick but I came close that time. It took about two days to get through. I woke up one night with my toes hooked on either side of the bed just to keep from rolling out! The only other time I remember it being that rough was coming out of the Panama Canal and headed to Aruba. We had 22-25 foot seas one night. We were one of a handful of tables that had people eating! The seas were so high that they would block the light as they went by the windows... it would be light, dark, light, dark...
The rest of the trip was calm sailing. The northern part of the trip was COLD. Bring warm outer wear and rain coats, too. It got nicer once we got into the Med. Cannes was beautiful... get a "REAL" croissant and chocolate there.
This is one of my favorite trips... you'll love it.