Presumably you would, or could, come in to Stranraer, and your parents might come in to Newcastle? Is there still a ferry from Ijmuiden to Newcastle? We took it 3 years ago after a few days in Amsterdam . . . .
In which case why not look at Dumfries and Galloway?? Lovely cycling routes and not as far as the Highlands - OK, not as High either. But not so much onward travelling for either contingent.
Dumfries has a lot of quiet road cycling as well as some hairy off road routes. If you do go further north, an area I enjoyed very much last year was Ardnamurchan and Loch Shiel - Resipole, round to Glenfinnan, then down the tracks to the east of Lock Shiel. Beautiful and still not as far as some parts.
Good luck
lewis