Hi - I'd like to say thanks to all those who made helpful contributions to this thread, and to add that I'm planning to copy - totally shamelessly - the route planned by McShroom. With the minor change that since I'll be in the area I'll have a go at getting to Cape Wrath, because reasons.
This obvs will be next summer or the year after, and the plan is to do it as a series of DIY by GPS 200k audax rides. Unsure whether to take tent or rely on B&Bs all the way.
But anyway - thanks to all. Esp McShroom who sent me his links.