i wish you all the luck with that one ......

...the DFDS ferry goes from Newcastle to Amsterdam not Rotterdam. In fact its Ijmuiden just north of Amsterdam on the coast. There is quite a bit of distance between the 2 cities in NL.
There are quite a few threads on this topic so have a search.
In a nutshell - Newcastle to Ijmuiden longest crossing and has tendency for stag do´s and booze cruise. A lot of pissed up Geordies having a day out in Amsterdam. Ferries are ancient and in my mind really crap for price. Lashing is done yourself.
Getting off in Ijmuiden can be a nightmare with traffic cameras and the roads are confusing to get to the main motorway.
I avoid this crossing like the plague.
If i go Oooop north then I tend to P&O to Hull which is better both sides to get to, cabins better and have recently been upgraded and the food on board is OK albeit expensive. Car deck is deck 7 if i recall and there is a specific bike lane on this one. Lashing to railings is easy BUT they do tend to jam the bikes in so beware of your panniers and then getting on and off.
Finally the one i use a lot simply because its closer to my work location is the crossing between Harwich and Hook of Holland.
A bitch to get to on both sides but the ferry is great. Shorter crossing, good cabins although food is not great.
happy to help on any more questions.