As regards the english ferries, I decided to go Dover to Dunkirk (£89 return) as it saved a lot compared to the Harwich to Hook of Holland ($176 return) that I looked at first.
I will have to brave the German motorways in an effort to save on costs
As Martyn says, the Greek ferries have fixed prices per person and per car just dependant on the season. For low season, I have paid £240. That is a return for me + my car, and a single for my friend (We are roughing it on the Airline seats). You also get the 30% discount on the return journey, as long as you book it all together.
I'll have a chat with you about it at the greek night in Huddersfield if you are still coming?