I know you'll have done some (or lots) of route planning. In case you find it useful, allow me to share this route from Camelford to Olveston (by M48)
https://ridewithgps.com/routes/31369072?beta=false
Camelford, Holsworthy, Winkleigh, Chawleigh, Tiverton (then canal), Wellington, Taunton (canal), Bridgwater, (Axbridge (west of), Strawberry Line and continue on NCN 26 then 410 all the way across Avon Bridge, wiggle via Hallen to M48x/Olveston (canals optional; gets one off the A38)
The first 75k and the last 100k are more or less the route I took for my last 2 hours Day 1 and all of Day 2 in 2015 and I've ridden the other roads (middle section (Chawleigh to Taunton)) quite recently.
Doesn't go to Weston, but visiting that town requires a fair detour: you have to go way back inland to cross the Congressbury Yeo river.
Good luck with your planning and preparations.