Beyond Selidor

Utter isolation and the absence of human habitation are not completely impossible to find in England. Dartmoor, Exmoor, the North York Moors and parts of the Peak District come to mind. In the wider UK, yes there are the Highlands and (less so, unless you find an uninhabited one) Islands, but even then there'll be planes overhead and evidence of prehistoric settlements.

Now; to a native Southern Englander like myself, the concept of total removal from all signs of humanity is a bit on the scary side. Scary, but exhilarating too.