I have cycled nearly all this route
If you want a reasonibly flat route until Cornwall, a lot of it can be done on Sustrans routes.
I'd do:
North Forland
North kent coast/Saxon way
Rochester
Dartford
South Bank of the Thames - Sustrans
Reading
Kennet and Avon canal
Hungerford
Bradford on Avon
Bath
Wells
Tarka Trail
Sheepwash
(Now it gets bumpy)
Holsworthy
Bude
Camelford
Camel Trail to Wadebridge
Lands end
(avoid the A30/A303 - it's motorway in all but name)
The most extreme easterly point of the island of Great Britan is Lowestoft Ness in Suffolk. about 500 miles from Land's End.
It is a non-intuative fact that the most westery point is Corrachadh Mor a headland on Ardnamurchan in Scotland. It is about 30 miles further west than Land's End. We imagine GB to be a reverse L but actually it is much more lopsided than you think...
It would be a fun tour to visit all the most extreme compass points of GB perhaps...
NORTHunnet Head
WEST:Corrachadh Mor
EAST:Lowestoff Ness
SOUTH: Lizard Point
I did this ride in 2007 from Land’s End to North Foreland. It was approximately 530 miles and I did it in 9 days so was fairly easy going at 50-60 miles a day on a touring bike carrying all my kit. This included going around the south coast of the Isle of Wight instead of north around Southampton, which I would recommend as a much nicer route. The Lighthouse at North Foreland is the perfect ending point as previously mentioned!