I don't need to point out to you that the A-roads would be very quiet at night, do I!! A day start from Land's End isn't obligatory: a FNRttC style start at midnight would be more... er, poetic? adventurous? ...as well as missing the worst of the traffic on Cornwall's A-roads.
I had given this some thought. It's not the first eighty miles to Launceston that bothers me so much as the Launceston to Exter stretch (once through Exeter the road on to Bridgewater is a lot of fun). It seemed to me (and this isn't a plan, merely a thought) that an early hours start from Launceston, leading to an extended breakfast in Exeter, an extended lunch in Taunton and an extended high tea in Bridgewater might be good....
I've not really thought this through, but....
Lands End to Launceston 79 miles. Not bad. Traffic west of Camborne isn't light, and there are some do-able diversions which reduces the travel on the A30 to about 50 miles
Launceston to Bridgewater 79 miles - early fast start, latter half of the day very pleasant
Bridgewater to Moreton in Marsh 91 miles - through Bristol which is interesting, but slow, and then on to Cirencester. Lots on the A429 which I don't know anything about
Moreton in Marsh to Gunthorpe 83 miles Fosse Way. The latter part of the day would be quite wonderful. Gunthorpe is on the River Trent and marks a kind of boundary....
Gunthorpe to York 75 miles. A lot on A-roads, but southerners will be surprised by the lack of traffic
York to Tow Law only 70 miles, but the last 15 miles is tough
Tow Law to Jedburgh 66 miles but very, very tough
Jedburgh to Glenfarg 79 - early start and breakfast in Old Reekie. There's a sweet diversion through Nether Blainslie. The road to Glenfarg is superb.
Glenfarg to Kingussie 82 - a delight
Kingussie to Dornoch 82 - even more of a delight
Dornoch to John O'Groats 79 - transcandental cycling happiness, with all your hopes and dreams rolled in to one ecstatic ball...........
Of course you're right about riding at night. It would be nice to include a night ride or two, but it would leave people feeling a bit knocked out afterwards.