Two couples will be spending 3 weeks in Great Britain. After 5 days in London we will spend a week in Lancaster at a timeshare doing day trips. Then we head to Edinburgh for 2 nights. We then have 3 nights unplanned then 2 nights on the Isle of Mull and then 2 nights in Glasgow. We will have a car. We are looking for suggestions for the 3 unplanned days. Someone suggested that we spend a night in the Orkney Ilands. Is this doable? suggestions? we will be traveling in July if that makes a difference.
Based on that you are doing Orkney is doable, but remember Scotland is a third of the island of Great Britain, that Edinburgh to Thurso for the ferry is almost all of that third, so that can easily take a full day and getting down to Mull would also take the best part of a day from Orkney, plus you will have the ferries to take into account.
It is doable but at a push. I would suggest looking somewhere else, possibly the north east or north west before heading to Mull. To put it in context, I am only slightly further from Thurso than you would be in Edinburgh, I'd stop on route overnight to Thurso to get the ferry. Coming back is different as long as you get to the ferry at in Orkney at the right time.
If you really want to go to Orkney, I would suggest flying. Fly from Edinburgh to Kirkwall. Spend two nights there--rent a car. Then fly to Inverness--pick up a car and drive to Mull--which is lovely!
If you are looking for other suggestions, you might just take a slow drive north up the center, and then west to Mull. You could go up toward Ballater, then back west again via the A939 to the A95, to the A9 and then west on the A86 to the A82 and Fort William and then south on the A85 to Oban where you catch the ferry to Mull.