For cheaper flights you may try Air One, one of the few Italian low-cost airlines http://www.flyairone.it/

Or, if you don't mind leaving out Rome, re-checking your baggage and changing airports, Ryanair has very cheap flights from London Stansted to Palermo http://www.ryanair.com/ , while Meridiana http://www.meridiana.it/en/index.html departs from Milan Linate to Palermo as well for 100 eur return trip.

Bye Paolo Trieste, Italy