If it helps, my parents always join us in Ibiza by coming overland/sea. (Mum is terrified of flying you see)
They go from London Waterloo to Paris Gare du Nord by Eurostar.
Then a taxi to Paris Austerlitz
Then the Paris-Barcelona 'HotelTren' train overnight which lands in Barca at about 0800 in the morning. Then a local train to Valencia, then the Acciona fast ferry to Ibiza town.
Setting off mid afternoon on one day, they arrive at Ibiza town at 7pm the following evening, so about 27/28 hours. My parents actually enjoy the trip (me, I prefer 2 hours on a plane) and treat is as part of their holiday, the Hoteltren is quite nice as you can get your own en-suite cabin, and dinner on board so it's all quite civilized - though it all adds up, cost-wise.