If it says "cruise only" then it does not include the flights. If it says "fly cruise", then generally the flight is included and often there is a choice of departure airports. However read the small print to make sure. Note that for Carribean cruises, P&O vary some of the smaller regional airports so that over the season there will be at least one cruise from an airport near you, although there is always Gatwick and Manchester.
The small print also says that they do not guarentee direct flts - so London - Rome may route you via Frankfurt.