National Cycle Network info here:
http://www.sustrans.org.uk/ncn/map/national-cycle-network
Printed maps obtainable for payment from here:
www.sustrans.org.uk/shop/route-maps-guides
Just looking at ordinary roads though . . .
Lowestoft > Diss > Thetford (or Bury St Edmunds) > Cambridge > Bedford > Aylesbury > Oxford > Cirencester > Bath > Wells > Glastonbury > Minehead > Barnstaple > Bideford > Bude > Lands End.
Returning . . .
Lands End > Helston > Truro > Launceston > Exeter > Honiton > Wincanton > Andover > Newbury > Reading > Maidenhead > Aylesbury > Dunstable > Luton > Baldock > Royston > Newmarket > Bury St Edmunds (or Thetford) > then as outbound to Lowestoft (or via Norwich if you insist upon not retracing any part of your route).
Those are just a few first thoughts (and, as I've been up for over 26 hours now, probably not very well put together) but they might at least give you something to work on while I catch up on a few hour's kip!