It's by ferry I'm afraid - being an island we're a 'captive' audience!!!
I went on to the web this morning to check prices for that weekend and crossings (one way) ranged from £173 to £402. When booking a return trip the closer to the weekend either way the price goes up so out Thurs and back following Weds the total best price is £350. Go out Thurs last ferry and return last ferry Sunday, best price is £506... On top of that by using the late ferries you HAVE to book a cabin - cheapest is £37pp.
Then we've got fuel, acommodation etc.
Look outside the norm a little and go Dover Dunkirk return with Norfolk Line ferries and a car and trailer is £ 134.00 return going out Friday evening and returning Monday morning. They are a great Ferry Co and are so much cheaper than the P & O's and the likes across to Calais.