Finding the best flights from the UK to Ibiza is difficult, with so few airlines travelling there in winter, and so many in summer.
Winter: There are currently no direct flights between the UK and Ibiza between the end of October and March, after Ryanair pulled its flights in 2011. In winter it is possible to fly there on a variety of routes including London to Madrid to Ibiza; London to Majorca to Ibiza; and London to Barcelona to Ibiza plus routes via Germany.
Be prepared to spend most of a day flying. However we've discovered some good routes with quite civilized flying times and at reasonable cost. Easyjet, Ryanair and Spanair are all worth checking. The site edreams is good for internal Spanish connecting flights. Watch out though as it charges a £20 administration fee unless you use a prepaid Mastercard credit card.
Summer: A large number of direct flights are available from Easyjet, Thomson, Ryanair, Monarch, BA and others. Check the aggregators or check direct with the airlines and holiday companies. It can be cheaper to book a package rather than just flights sometimes, even if you don't use the accommodation.