Lindesfarne. This takes me from the Welsh mountains, and seashore of my first 4 things to do across the Island of Great Britain to the Northern English East Coast just south of Scotland in Northumberland.
After a walk of anywhere from a few days, to over a week - depending upon where one might choose to begin - the final day of the Lindisfarne Pilgrimage is a three mile walk across muddy, sometimes dangerously muddy path across the end of St. Cuthbert's way on the "Pilgrim's Way" to The Holy Island.
"Visitors wishing to walk between the mainland and the island are urged to keep to the marked path, check tide times and weather carefully, and seek local advice if in doubt. Visitors driving should pay close attention to the timetables prominently displayed at both ends of the causeway and where the Holy Island road leaves the A1 Great North Road at Beal. The causeway is generally open from about 3 hours after high tide until 2 hours before the next high tide, but there is no substitute for checking the timetables for a specific date, and the period of closure may extend during stormy weather."
The Lindisfarne Pilgrimage makes my #5 on things I want to do before I die. Anybody want to go puddle jumping with me? Of course, the pilgrimage ends with Lindisfarne's famous mead. Now that's a fine way to end a medieval trip.