Steve,
The first question that occured to me after reading your most recent post is given your vacation time and the cruise departures, could you travel before July? I'm assuming the cruise you're planning is the Princess "Southampton Roundtrip", which appears to have departures on June 23, July 17, August 10 & August 22. July and August are the busiest months in Europe, so the June 23 departure might be preferable?
Regarding the airfares, I'd suggest starting to watch these early in the New Year, as the airlines likely won't have their 2010 schedules finalized yet. In addition to the "usual" sites such as Expedia, also check the airline sites and perhaps speak with a Travel Agent.
I'd highly recommend avoiding Alitalia, especially if you'll be travelling with checked luggage (you might not see it again until you get home!). Check other posts here, as they have a bit of a "reputation".
As your cruise returns to Southampton, you might consider EasyJet from London Gatwick to Rome/FCO instead. You'll have to deal with their luggage limits and the "ONE carry-on bag" policy. I did a brief check and found fares as low as £32.99 in March (which is as far as the schedules go at this time), which I believe is inclusive of all taxes and fees. "Late bookers" usually pay more, so if you want the cheapest prices, you'll have to monitor their website. NOTE that fares with budget airlines are often non-refundable and non-changeable once booked (although some will allow change for a fee). There may be extra fees for baggage if you're "over the limit", so be sure to pack light!
Cheers!