Great idea - it's great over there and the ferry is super.
How about taking the train to Tweedbank and starting from there? You could take a fairly flat way over to Etal (follow the Tweed on the other side from the main road) then make your way to the NCN 1 on the coast.
The cyclepath from the ferry port to Newcastle train station is relatively good for here and mainly off-road - but somewhat hiller and glass strewn than in the NL. It takes longer than you'd think, especially with panniers. It's also pretty hard to find a good way between the river where it starts and the station.
The Fietsnoop app is brilliant for route finding (point to point Dutch style) and it will download gps tracks (which is good if you forget to look out for the numbered points). The maps are good for overall planning but the app is more up-to-date and zoomable.
Take bells! And make sure your brakes work (for the towns - with mad fellow cyclists). Also look up in advance what the street signs mean. It is worth knowing the difference between cyclists permitted and prohibited.
We may be going as well sometime in the spring (or to England or to France or ....)