As for how to go about it ...
I'm lucky in that I have Memory Map OS mapping for the entire UK at 1:50,000 scale, but I can also zoom out and view the country at different scales.
I start off drawing a straight line from the start point to the finish point. If there is an obvious quiet, scenic route close to that line, that's the route I plot. If not, I scan the area either side of the line to see if there is are places I'd like to avoid, and places that I'd like to pass through. Obviously, there is no sense in heading for some lovely village only to then have a 20 mile detour to avoid the big city beyond it!
Having worked out a rough route, I then start looking at distances and how much climbing is involved. That gives me an idea of how much leeway I have. If I can afford to do some meandering, then I'll choose the nicest-looking route between the Points Of Interest. If I am going over-distance, then I'll take a more direct route between POI, even if that means some slightly busier roads. I do my very best to avoid busy A-roads where possible, except for short stretches which are sometimes necessary to get from one nice place to another.
It looks to me that if you were prepared to do ride about 80 meandering miles from Richmond to the ferry port at Portsmouth, then you could come up with a really interesting route, about 75% of which would be on quiet/quietish roads. Good luck!