Stonehenge is best visited first, approached from the north on the track from Larkhill which crosses the A344 by the car park & visitor centre (subway under road between car park & Stonehenge). I'd probably go through Andover, Tidworth, Bulford to Larkhill, unless you want a longer route.
From Stonehenge, retrace to Larkhill, left (west) to Rolleston Camp, then north past the Bustard Inn and over Salisbury Plain. Drop down Redhorn Hill, head east on the A342 for just over 1km, turning north at Chirton through Marden, Woodborough and Alton Barnes (white horse).
At the canal just before Alton Barnes, there's a track by a timber yard that goes 400m to a fairly decent pub with a campsite.
From Alton Barnes, up over the hill, then first left through East Kennet,West Kennet and cross A4 to Avebury. Depending on traffic, divert along the A4 to look at the Sanctuary and/or Silbury Hill. If you are feeling adventurous, from the top of the hill after Alton Barnes, it's quite interesting to follow the Wansdyke for a couple or three km west before dropping down to Beckhampton and into Avebury the other way. Tea shop in the NT vistor centre, pub in middle of village.
Going north from Avebury, either follow the A4361 for 6km or so, then left to Winterbourne Bassett & Clyffe Pypard (good pub with
YHA bunkhouse attached). The non-main road way out of Avebury is up onto the Ridgeway and follow that north to Hackpen Hill, then Broad Hinton & Clyffe Pypard.
After that, it really depends which bits of the Cotswolds you fancy seeing. Note however that the Cotswolds aren't particularly well endowed with camp sites.
I'll suggest the following for a slightly roundabout "seeing the Cotswolds" route:
Tockenham, Tockenham Wick, Callow Hill, Minety, Ashton Keynes, Cerney Wick, Down Ampney, Meysey Hampton (pub), Quenington, Coln St Aldwyns, Bibury (tourist honeypot), Ablington, Winson, Cold Rogers, Coln St Dennis, Northleach, Farmington, Bourton on the Water (bigger honeypot, fairly nice campsite by the fish farm), Lower Slaughter, Upper Slaughter, Cotswold Farm Park (tea shop, campsite), Snowshill, Broadway (several tea shops, best pub is on the right about 300m before the main street), Childswickham, Elmley Castle, Pershore, and then cross the Severn at either Upton or Worcester.
For a more direct route, I'd go (from Minety, as above), Somerford Keynes, Ewen, Coates, Sapperton (pub), Miserden, Whiteway, (campsite), Birdlip, Brockworth, Churchdown, Innsworth, Down Hatherley (A38), Norton, Haw Bridge (campsite - more appealing than the one at Wainlodes Hill a bit earlier), Tirley, Forthampton, Longdon, Upton-on-Severn. Near Upton the best campsite is the big-but-good C&CC site at Blackmore End just N of Hanley Swan.
North of Upton I'd skirt Malvern via Guarlford, Madresfield and Leigh Sinton, then Alfrick, Knightwick (pub), up Ankerdine Hill (steep), Martley, Shelsley Walsh, Pensax & on via Cleobury Mortimer.
Alternatively, from Alfrick, go Linley Green, Bromyard, Tenbury Wells, Ludlow
(all a bit off my patch)
On Salisbury Plain, if it's been wet there will be big puddles. Near the south end of the track there are some bits of remnant tarmac, which can make for sharp edged potholes that will have you off if you hit a submerged one too fast (BTDTGTTS).
If you bring the Wales & West Midlands OS 1:250,000 sheet 6, route discussion at Wyke Down would be possible.
[edit] any reason for asking here rather than on YACF?
