Calne onwards, I'd go Bishops Cannings, Etchilhampton, straight over Black Heath to the Bustard (assuming it's a weekend and the ranges are open), Shrewton, Stapleford,
Great Wishford,
Wilton, Salisbury or Old Sarum, Pitton, Middle Winterslow, Kings Sombourne,
Winchester, Morestead,
Meonstoke, Soberton Heath, Southwick, Pompey
Autoroute shows 176km from Nailsworth, going via the A360 'cos it doesn't have Black Heath on. You can go Calne, Pewsey, Amesbury, Wilton, which ups the validated distance to 185km, with nice roads close to the main roads for not much extra.
So, assuming a 200km route that will pass validation, and having a play in Autoroute, how about this for a suggestion...
Controls:
Nailsworth, Pewsey, Fordingbridge, Winchester, Meonstoke, Pompey :- Autoroute reports 200.4km
("Pompey" being located by the entrance to the France/Spain ferryport)
The actual route goes N, Avening, Long Newton, Malmesbury, Sutton Benger, Calne, Bishops Cannings, Pewsey, Upavon, down A345 to Salisbury & A338 to Fordingbridge, B3078, Bramshaw, Romsey, Braishfield, Winchester, Morestead, Corhampton/Meonstoke, Soberford Heath, Southwick, Pompey.
Problems:
It comes up 3km short if you can't get a control in Corhampton/Meonstoke (probably a postcard job?). Following the Avon valley lanes instead of the A345 between Upavon and Salisbury adds 5km. (via Fittleton, Bulford, Amesbury, Wilsford, Woodfords)
Following the roads on the east of the Avon instead of the A338 adds 5km (via Aldebury, Downton, Woodgreen, Breamore)