In 1995, before leaving his much-loved home in North Yorkshire to move back to the States for a few years with his family, Bill Bryson insisted on taking one last trip around Britain, a sort of valedictory tour of the green and kindly island that had so long been his home. His aim was to take stock of the nation’s public face and private parts (as it were), and to analyse what precisely it was he loved so much about a country that had produced Marmite