London Outer Orbital Path

The London Outer Orbital Path — more usually the "London LOOP" — is a 240-kilometre (150 mi) signed walk along public footpaths, and through parks, woods and fields around the edge of Outer London, England, described as "the M25 for walkers". The walk begins at Erith on the south bank of the River Thames and passes clockwise through Crayford, Petts Wood, Coulsdon, Kingston upon Thames, Uxbridge, Elstree, Cockfosters, Chingford and Upminster Bridge before ending at Purfleet, almost directly across the Thames from its starting point. Between these settlements the route passes through green buffers and some of the highest points in Greater London

More information on the Wikipedia page.