Hi, We've done it a few times, we've tried Brindisi and Ancona, but we found that Anek lines from Venice suits us best. Venice is about 1000 miles, give or take a few from Calais depending which route you take and the ferry will take approximately 25 hours. Unfortunately in April you won't be able to travel direct to Corfu and you will have to go to Igoumenitsa on the mainland and get another ferry from there, they run regularly, are not overly expensive and only take about 1 hour and 30 minutes, also there's no need to pre book.
Good luck with the journey and enjoy.