If you go from Zolder->Edinburgh via Ijmuiden-Newcastle it's 559 miles less 303 for the ferry = 206 miles, but you won't arrive into Edinburgh until Thursday lunch. £243 for 2 cars, 2 people sharing a cabin = 121.50 each + 40 quid for a tank of fuel.
If you go from Zolder->Edinburgh via Chunnel it's 674 miles, less 33 for the chunnel = 641 miles, and you could be home for around Wednesday 10pm if you left Zolder at 8am. £91/car + 125 quid for the fuel.
So it's the usual trade off - the ferry is cheaper and more relaxing, but it takes longer and you have to put up with the "entertainment"
Cheers,
Robin