Hi Antonio
You can get the overnight ferry from Rotterdam to Hull (in the North Of England), disembark in the morning with a hangover then have a pleasant ride up to Scotland.
Best to make an early reservation as the sailing gets busy at that time of year. You need to book with P & O ferries. You will get a cabin but may need to bring your own ratchet straps to secure your bike as they tend to give you crappy bits of greasy rope.
You are looking at a journey of around 700kms from Hull up to Spean Bridge depending upon which route you take. If you have time, I would take the scenic route and take in the Yorkshire Dales, Pennines, Borders, Trossachs and Cairngorms and do it over 2 days. Some spectacular rides through hills and mountains. Take your waterproofs and plan your fuel stops (if in doubt fill up!) in the Highlands.
The alternative is to shoot up the boring A1, then the A9 if you are short of time. School holidays will have started by then so I would be tempted to stay off the busy main roads and scrub in the edges of your tyres.
Let me know if you decide to go and need a hand planning a good route.
Regards
Guy
P.S You can also get a ferry from Amsterdam to Newcastle which lands you further up North but I have never tried this crossing so I do not know what it is like.