Ben - I hope you take this comment in the constructive spirit it's meant, and a bit OT. I have ridden nearly of the roads you describe, normally as part of 200+km rides, one of them at the same tim(ish) as you plan (leaving Penzance at 1845 and doing 125km through to midnight). I can't tell from your map which "quieter country roads" but at that time of night the roads are pretty quiet, even the main roads (not suggesting A30). Doing 'distance' using "quieter country roads" is a challenge, and at night is a significant challenge, both road width/surface/bendiness and navigation (and one you don't need), and not really safer either, at night.
I offer you an
alternative A 'Part 1' route (Land's End to Warminster) going north of Bodmin Moor (your route is seriously hilly around Minions) through Launceston and Okehampton. Also be aware that the hills NE of Crediton (to Bickleigh/Tiverton) are normally worth going round (to the SE). This route has nearly 900m (3000ft) less climbing and the same distance. I can get another 1000ft off by going ENE from Wadebridge through Holsworthy, and not dropping down/across to Okehampton (acknowledge chez "Ian's brother"), on this
alternative B 'Part 1' route (Land's End to Warminster) variation. I have ridden all these roads (2015/2016).
If you want, send me a link to your routes and I'll give you detailed comments (and places which might be open during the first 200km ie Saturday dawn).