Ferries to Holland
Your choices from the UK
The choices for the ferries to Holland are: Hull to Rotterdam (P&O Ferries), Harwich to Hook of Holland (Stena Line), and Newcastle to Amsterdam (DFDS Seaways). This gives a good geographical spread wherever you are in the UK.
Scottish passengers can choose the Newcastle or Hull ports, but an alternative to consider may be the Rosyth to Zeebrugge (Belgium) route operated by Norfolkline.