Hi Russell,
I work near Rotterdam - about one week in three - and whilst I normally use the Dover - Calais crossing, I have used both the HSS from Harwich to Hoek van Holland, and the Hull to Europoort services.
The HSS ferry to Hoek van Holland is excellent, about 4 hours. But if the weather is bad, the catamaran makes an unusual yawing motion.
I've used the Hull to Europoort overnight service, sleeping in an inner cabin - obviously no windows - and it's OK but try to get a cabin with an integral toilet / shower.
No comments on the Newcastle or Rosyth ferries, except that Zeebrugge is in Belgium and about 3 hours drive to the Rotterdam area.
Where are you planning on staying?
I can recommend the "Delftse Hout" campsite for anywhere between Rotterdam and Amsterdam. Easy walk into Delft, good railway links - don't drive into Amsterdam or Leiden. Very popular site - you will need to book.
Just South of Rotterdam, and further West is the old city of Brielle. You could take a look at "Camping De Krabbeplaat" or any of the coastal camp grounds between Brielle and Middelburg.
This coastal region is very popular with Dutch and German holiday makers and you may need to be flexible.
Robert