This week I took the ferry from Harwich to Hook of Holland, and drove to Antwerp with some gear needed for work I'm doing out there.
See :- Ferry Trip Set on Flickr
Overall, I was really impressed:
Taking the overnight ferry from Harwich let me get a good night's sleep and I was in the Office at Antwerp by 10.15am the next day.
Coming back was more interesting, but the real benefit of the ferry is actually travelling while you're asleep.
Accommodation in the cabin and public areas was excellent and meals/refreshments were much better value than airports.
Overall cost is much less than flying from Norwich, though I think I'd probably leave my car at Harwich and take trains in the Netherlands/Belgium if I go again.
The drive wasn't bad but you have to keep your wits about you because of the number of times you have to "keep left" or "keep right".