ferries

Hey, my best friend an I are planning to do a backpacking trip in northern france this summer. We also wanted to spend a few days in london and very very tentatively considering Dublin. We were really hoping to take a ferry across the channel, just for the experience, but it seems like this is extremely expensive. Does anyone have any thoughts or info about ferries (I was really hoping to go Granville to Jersey to Portsmouth) or know anything about discounts?? Thanks.