If you are coming from The Netherlands (Or any EU country) you can get the train to Hoek van Holland Strand. From there you can walk to the ferry terminal.
Depending on what boat you get (Day/Overnight) WarZone will be doing pickups from Harwich at 06:30 and 19:45 on Thursday.
The earlier you book, the cheaper the boat becomes. At the time of publication, an on foot ticket costs £78 for a return.
For information and booking, go to Stenaline.co.uk