I did Denham-Stornoway-Faroes last summer in a PA28-180. We stopped at Carlisle en-route. Initial routing was Harrogate, then over the Pennines at FL65 IMC. After Carlisle we routed west of Glasgow and up through the islands. My companion is based on Stornoway and he says this is the best route, he's tried several. We had a Radar advisory up to Carlisle, then FIS from Scottish Info thereafter, they're really good. The route up through the islands is absolutely stunning, we were partly IMC but when there was a break in the cloud it revealed spectacular views.
If you can, go up to the Faroes and try the waterfall approach.
TheOddOne