When is this all taking place?
That ferry site seems pretty good. Here's another
another one - very similar.
According to above sites, you have to book a cabin on the ferry since it's overnight. It looks like you leave around 17 30 and arrive and 9 30 am. This may be cost effective in the sense that you are taking care of that nights accomadation plus transportation.
If not, you could fly out of Liverpool or Leeds. The major places like ryanair stuff don't have direct flights from there to A-dam, but if you look at
it'll direct you to whatever cheap flights go between the two. Since Manchester is near Liverpoo/Leeds, you could fly to A-dam without having to go through London. Not sure which would be more cost effective. The ferry seemed a bit pricey, but I didn't know your stats (dates, age, people in party).
also has some sites for flying out of England.
This is all I found on the web. Maybe someone has some personal experience they can share!