Upvote:4
You can take the Caledonain Sleeper from London Euston to Fort William, get down at Inverness(nearer to Orkney) and then hire a car from there. And believe me the train journey is awesome in the morning. The Scottish highlands are a treat to watch in the morning sun.
Upvote:5
There is a ferry from Aberdeen to Kirkwall (Orkney) and Lerwick (Shetland), operated by Northlink Ferries. It calls at Kirkwall three or four nights a week.
Northbound, it is not really overnight - it departs Aberdeen at 1700, arriving at Kirkwall at 2300. The return is overnight, depart Kirkwall 2345, arrive Aberdeen at 0700. Note you will have to check in about 1 hour before the departure.
Driving, it is a shorter journey from London to Aberdeen, about 9 hours (according to Google Maps). Instead of about 11 or 12 hours from London to Scrabster or Gills Bay, to get one of the shorter ferry journeys.