Whenever I do a long tour (well, that's twice, so far), I do it on paper. Get a large scale road atlas, to give you an overview, a list of places to act at reference points (I'm guessing you'll need at least one overnight stop too?). Then look at the appropriate smaller scale maps to find the backroads between those points. You could buy all the OS landranger maps, but that'll get expensive. With a bit of patience I reckon you could do it on Streetmap.co.uk, or the OS Getamap site, shifting between map scales to get your bearings. If you already have some bits that are ok, you could just look at the bits that bother you, for alternatives.
A really good organiser would then drive the route to check it out (esp things like nasty junctions) - sometimes you can't get it all from a map. If the route follows cyclepaths, of course, you can't drive it, but you could take a bike by car and then explore the off road bits - if you're going to be on road bikes, for example, a really crappy surface might be an issue. Also, if you drive the route, you might find info that's useful but not on a map (like a pavement bike path along a section of main road that might avoid a long detour).
Using maps might also allow you to cut out the odd hill, if you can read contours.
