The River Thames doesn't just flow through London, but as the longest river in England, begins in a field in Gloucestershire and winds for 215 miles through 9 counties until it reaches the North Sea.
In 2015 I walked its entire length, beginning at Southend-on-Sea where the river ends, tracing it right back through the Essex Estuary in to London, down to Reading, up to Oxford, then along in to the Cotswolds. The whole adventure took about 3 weeks, but I condensed it down in to a headache inducing 5 minute video entitled 'From the Sea to the Source - Walking the River Thames'.
Bowl Of Chalk
Bowl Of Chalk based shenanigans.