Ridgeway / Icknield way
I have started to walk the Ridgeway backwards starting from Invinghoe Beacon in Buckinghamshire, heading south west initially though the Chilterns, crossing the Thames at Goring and heading on through the Wessex downs to Overton Hill at the southern end of the Marlborough downs in Wiltshire. There is a national trail link here.
There are further links at the Friends of the Ridgeway and the Great Chalk way which documents the paths from the Dorset coast to the Norfolk coast. Plus a Trailblazer guide for it here and the Circerone route map booklet Walking the Ridgeway. Also like Stephie’s blog on the walk.
Thames Path
The river Thames is claimed to start at Trewsbury Mead in the Cotswold near the villages of Coates and Kemble. There is a national trail called the Thames path that starts there.