Sail from Harwich, UK to Hoek van Holland overnight on Stena Line's luxury superferry Stena Hollandica, sailing at 23:00 & arriving at 08:00 next morning. Remember that Dutch time is 1 hour ahead of UK time - so about 8 hours.
Cheapest one-way ticket from Harwich is about 39.5 GBP, but for overnight travel you need to book a cabin as well which is 35 GBP. For day travel (leaving Harwich at 9:00 am and arriving Holland at 5:15 pm), the ticket is again 39.5, but the cabin rate is discounted 17.5 GBP.
I paid about 90 GBP for my ticket and it included train Ticket from London to Harwich+Ferry Ticket+Cabin Charges+Train Ticket up to Amsterdam. A great value indeed. Plus by travelling overnight you save on Hotel Room costs for one night.
Please do check out https://www.stenaline.co.uk/ferry-to-holland for complete booking information.