Agree about Avebury, no mass-tourism.
Take train to Falmouth and cycle the coast via Gweek, Mullin (Cove) and Penzance. Visit St. Michael's Mount, continue throug finest hills and coastal jewels near Mousehole and Lamorma to Porthcornu,visit the white beach and the Minck Open Air Theatre, avoid the tourist trap Land's End and cycle from St. Just to Hayle and Truro. Train again, to London or ...
And make a choice: Oxford, Cambridge and stay in the south and don't practice the must-do-syndrom.
London, Canterbury etc. etc.
(I cycled Cornwall two weeks ago, marvellous)
Arie
Have a pleasant trip.