Depending on how much time youŽd be willing to spend for the journey (and which season or rather how "sensitive" you are regarding weather/temps) IŽd choose the ferry between Newcastle and Amsterdam (Ijmuiden).
There are no ferries to the Netherlands from north of Newcastle so you need to get there either by train/coach/car.
Additional info:
https://www.dfds.com/en-gb/passenger...n/bus-transfer
The ferries are overnight leaving Newcastle 5pm arriving Amsterdam 9.45am.
Ferry operator is DFDS (if you decide in favour the ferry crossing IŽd recomend to book on their website since thereŽs no advantage via OTAŽs)
Link:
https://www.dfds.com/en-gb/passenger...stle-amsterdam
In comparison going by train all the way via London (change of train+railway station required) the overall travel time may not be much different from a ferry crossing ex Newcastle.
You can either choose to travel to London over night by sleeper or daytime train and continue from London by Eurostar.
Unless you find a really good deal IŽd expect the train only trip to be generally more expensive ...
Btw.: The proposed ferry link Rosyth,Scotland to Eemshaven,Netherlands has not materialized so far