If trying to make good progress, I'd probably aim for hitting the A30 around Hayle at 6am on a Sunday, so maybe departing at 5am (sunrise in summer)? Then keep with it until the traffic gets too bad, which quite possibly gets you as far as Launceston, then A388 -> Okehampton -> B3215 -> A3072 -> A377 -> A3072 -> Tiverton?
Sunrise next weekend at Land's End is 0513, but it'd be light enough to ride from 0330. Just depends on getting out of bed early enough!
Launceston is 125km from Land's End on the A30. From Hayle it's dualled ALL the way. Rider's in a hurry do, against advice, do what you suggest and start at 5am , and hopefully make Launceston bypass by 9.
From Launceston, the A388 doesn't go Okehampton, it just goes out back to the A30 (dualled). Then you have the 'old A30' road nearly to Okehampton, which is fine.
And the 'old A30' then takes you all the way to Exeter; where turn NNE on the 'old' and new A38 - if you're "in a hurry". Michael Broadwith was 'in a hurry' and went via Exeter.
I'm going to
hazard a guess that you have not ridden the A3072 from Crediton over to Tiverton, but are a 'map warrior'.
The climb over the Cadbury Hills to Bickleigh on the A3072 is 13.8km + 287m. Grim after 180km.
Route options
Okehampton to J27/M5 services (E of Tiverton on A39)
Old A30, through Exeter, and B3181 = 67.6 km +716 m I have no idea what taking the dualled A30 to Exeter would be, I'm not even going to measure it. Fine if you have a following vehicle all the way, protecting your a**/life.
"B3215 -> A3072 -> A377 -> A3072" -> [Halberton Road] = 60.4 km +836 m The A3072 goes over the Cadbury Hills from Crediton.
"B3215 -> A3072 -> A377" ->Crediton then [Thorverton, Bradninch, Cullompton] = 59.2 km +774 m This is the 'classic' end-to-end route which skirts the Cadbury Hills.